You're the one who is falling flat IMO.
To say that Bruce had no good reason for retiring, you might as well say "I didn't really pay attention to the plot of this movie". Because it was explained. There were no crazy freaks running around for Batman to tend to, courtesy of the Dent Act. After TDK...BATMAN was the villain of the city, not Joker anymore, not Two-Face, not some other crazy like Riddler or Penguin, noooo. It was Batman that was the villain. The Harvey Dent Act reduced the evil in Gotham down to small-time criminal stuff that the police can take care of without problems. Is Batman (the villain at the time) going to lurk around to take care of a random thief when he's not supposed to be exposing himself and making it seem like he's still doing good for the city??
First off, yes there are no more crazy villains (aka "freaks") after TDK and Batman is the only true villain but the point is that there SHOULD have been according to TDK. TDK brought up the idea over and over again that Batman is responsible for the "higher class of criminal" like the Joker being in Gotham City because his presence is what brought the Joker there. The Joker also states and alludes multiple times in the movie that his presence there is just the beginning - soon, more and more "freaks" like him will show up in Gotham City and will take over as Gotham's most dangerous criminals
not because of Dent (aka the ace in the hole) but because of Batman. Batman's presence has lead to the birth of this. He is what brought that "higher class of criminals" to the city.
"All these mobs, corrupt cops, and crooks want you out of the way so that things can go back to the way they were before when they had control over Gotham. But I know the truth. You've changed things. Forever. There's no going back." -Joker to Batman (not word-for-word but you get the point)
This whole idea about Gotham being changed from a city in control of the mob to a city full of "freaks" that continue to grow in numbers due to Batman's presence is an concept taken from The Long Halloween, one of the comics that heavily inspired Batman Begins and TDK (TDK a lot more; you can almost say TDK is a loose adaptation of Long Halloween but with Joker as the main villain instead of Holiday and with a different ending). Long Halloween did the same thing.
The Dent Act IS what I'm talking about when I say the arguments to rationalize what Bruce did fall flat. If you analyze the Dent Act and the events in both movies, you can see why it just doesn't work and why it wouldn't keep Bruce away from being Batman. Here are my two cents on why Batman quitting because of the Dent Act doesn't work:
1) It's a complete "deus ex machina" act. There is no such thing as an act that can eliminate organized crime, let alone cut down crime rates to small criminal stuff the police can easily take care of. Even New York in the 70's, which was almost as bad as Gotham in terms of crime and corruption, still hasn't fully fixed itself to the point where you can have low crime rates the police can easily take care of. The act maybe would have been ok in a different movie but for Nolan's realistic setting, it doesn't work. It would be about as effective as a bill that illegalizes prostitution and the marijuana industry - meaning they're not too effective since people still have easy access to both those things without getting caught.
2) Even if it would work, the act doesn't put an end to all crime. It just puts an end to organized crime, which means there are still TONS of criminals out there to be caught. Unorganized crime can sometimes be even more chaotic than organized crime because there is no one to hold Gotham's criminals by a "leash" and tell them where and when to act. They would need Batman more than ever if organized crime falls.
3) Gotham's organized crime falling is 100% irrelevant to the necessity of Batman. Like I stated above, TDK set up the idea that more and more freaks like the Joker would continue to show up due to Batman's presence and THAT is the reason Batman needs to stay. The mob would have fallen anyways according to TDK even if the act wasn't passed due to the arrival of the freaks. And even though there are people in Gotham like Harvey that are motivated and strong enough to fight against criminals of the city, there will be a "higher class of criminals" in Gotham that only
Batman can take care of. That is why the Joker managed to corrupt Harvey but failed to corrupt Batman. Because Batman can take it. And he can take it because he's "more than just a man" (going back to Begins) while Harvey was, at the end of the day, just a man which is why he was corrupted. Even though Harvey motivated the city to fight against its crime and corruption, Batman was needed to stay to fight any other people like the Joker showing up which once again, according to TDK, more of them would have been coming soon. The moment when he saw Harvey being corrupted by the Joker was also the moment he realized that only he is capable of being Gotham's guardian because he is "more than just a man" and also because he is a freak like the Joker (just a different kind of freak) thus only he can match up to people like him.
4) TDKR heavily implies Bruce quit being Batman right after he got home that night Harvey died. The act probably wasn't passed until much later so Batman was STILL needed for a while. Yes, there is the possibility he continued being Batman a bit after that night when Harvey died but I find that really unlikely because there isn't much in the movie to suggest/imply that yet there are tons of stuff that imply he quit right after that night - his leg injury (which was most likely received from his fall in TDK as they imply several times in the movie, which I call BS on TDKR for since he seemed completely fine in TDK but let's just say for the sake of argument that that is where he injured his leg even though it doesn't make sense), the last public sighting of the Batman being 8 years ago, a female from that kitchen at the beginning of the film stating "no one's seen Mr. Wayne in 8 years", and more. I'm pretty sure they also say the Dent Act was passed 8 years ago. So it's safe to assume he quit right after that night despite it not making any sense.
Those are my views on why the Dent Act falls flat and why Bruce doesn't really have a reason to quit. Feel free to agree or disagree.
Now, if you're a person who is adamant about Batman being active during the 8 years...which is what you sound like. (it sounds like you're mind is already made up & you think any story couldn't work unless Batman kept fighting crime after TDK ending). So if ur one of those ppl, i think they were vague enough so you can believe that Batman was around for a few years before the Dent Act kicked in until he retired for good. Sure he wasn't "seen since Dent was murdered" but if you're one of those people, u can believe that it was just in the public eye that he hasn't been spotted for a full 8 years. I don't agree with it, but if it makes you happy, go right ahead. Batman can be very low-key and deal with other rogues who are secretive. Riddler is a little more of an attention seeker but Bruce Wayne could deal with him behind the scenes, minus the cape & cowl. Who knows, maybe he's only been completely done for the last 5 years. Whether you want to believe in that or you take the 8 years literally, it should still work out.
If you want to believe that it's stupid nobody is stopping you. But don't act as if Bruce retired for no apparent reason.
Well, not much for me to say here. I addressed all of this in my above paragraphs.
You say that TDK was about Bruce coming to the realization that he must be Batman forever, and there's no way out of it. And he's more motivated than ever to be Batman. And there would be more freaks in Gotham since the mob fell. But none of those things are facts AT ALL. Those are YOUR views on the movie, not everybody elses and certainly not Nolans intention obviously. The end of TDK wasn't Batman realizing that he must be Batman forever, it was a realization that the symbol of the Batman must take the blame. Wanting more freaks in the city is just that and nothing more: a "want". Some of us wanted to see that but it doesnt mean it had to happen. We only thought this way because Joker predicted this, but at that moment he thought Two-Face was his ace in the hole. If Two-Face had lived, yes....im sure he would have started a gang and more freaks would come out and it could have led to an Arkham City situation. But Dent died, and Jokers prediction fell flat. The mob fell and that was that....Batman/Gordon covered it up and the Dent Act was passed.
The reason the Joker believed more "freaks" would have came into the city had nothing to do with Dent. It was all because of Batman. He believed that Batman's presence in Gotham opened the door for that higher class of criminals to step into the city. The Joker himself is one of those higher class of criminals that stepped into Gotham due to Batman's presence. I wasn't referring to when he said Harvey was his ace in the hole either. I was mostly referring to this:
"All these mobs, corrupt cops, and crooks want you out of the way so that things can go back to the way they were before when they had control over Gotham. But I know the truth. You've changed things. Forever. There's no going back." -Joker to Batman (not word-for-word but you get the point)
Yes, it was a realization that the symbol of Batman is capable of taking the blame but it was also a realization of the fact that only he was capable of doing what Batman does and that there is no on-par replacement for Batman out there thus he must accept that Batman is who he truly is and who he has to be forever (or at least until he's an old man and someone else takes the mantle Batman Beyond style). Throughout TDK, Bruce was trying to look for a replacement for Batman. His original plan, from the moment his parents died up till the TDK ending, was to be Batman for a temporary amount of time until he would fix his city and/or find a replacement then he would hook up with Rachel and live a normal life. That was the illusion he lived under. He thought that was possible. He found a solid replacement in Harvey's character. Even went as far as to support his campaign in order to get Harvey to replace him. Then the Joker comes in, corrupts Harvey, and brings him down to his level. He also tries corrupting Batman but fails at it. What Bruce learns from this is that there is no replacement for him. Despite how good a man can be, he can easily be corrupted by a higher class of criminals (a.k.a. supervillains) including Harvey. Because as long as someone know how to get to you, you're weak. That is the reason why Harvey was corrupted and Batman wasn't. That is what separates Harvey from Batman. The reason Joker failed to corrupt Batman is because Bruce is "more than just a man" (going back to BB) while Harvey was simply just a good man and thus, he had limits. With Harvey's fall, Bruce realized that he HAS to be Batman forever and that there is no escape from this responsibility. Batman is the only force out there that can deal with types of criminals like the Joker and the life he wanted to have by quitting as Batman and having a family with his loved one was just an illusion. It's one of the most brilliant character arcs ever done with Batman IMO.
I am willing to admit this may all just be my perception of the movie. However, I really doubt they didn't intend to have at least most of this stuff in there especially since TDK is heavily influenced by The Long Halloween and TLH touches on this stuff over and over again throughout the book (especially with the idea of the "freaks" taking over Gotham from the mob since Batman's arrival). Just because he doesn't verbally state at the end of TDK that he must continue to be Batman forever doesn't mean it isn't there. Not every message and theme is shoved down our throats through the dialogue, which is something the Nolan Batfilms constantly get accused off by some people.