Yes...seriously.
You can't really argue that the 8 year timeout was made necessary by the events of BB and TDK, since both films heavily pressed the theme of escalation. You say that the LOS and the mob were the only threats in that Gotham, but I'm sure I remember The Joker, The Scarecrow and Zsasz making an appearance, too, as well as the contents of Arkham Asylum. Didn't The Joker make a monologue foreshadowing the immutable nature of Batman?
Only major threats, yes. The Joker is his own animal but really he's still a part of that internal 'mob' threat. He's not a Falconi or Maroni, but at the end of the day I'd consider him another mobster (just a much more sadistic, fiendish, clever one). Scarecrow was just a pawn of Ra's, and Zsasz' appearance was nothing more than a ten-second fan shout-out.
The Arkham inmates are criminals but they're not any sort of masterminds like Joker, the mob families, or the League. They're just criminals in the city. Not 'villains'. And the rooftop scene in BB tells us that Gordon is working on recapturing the escaped inmates anyway. Yes, many of these criminals become a part of Bane's army, but again they are dispensable. They'd remain locked up if not for Bane.
The decision was clearly made to make a third film, despite the impossibility of The Joker's intended return. So we got a sort of reworking of The Dark Knight Returns instead, with No Man's Land and Knightfall stirred in. The former was followed closely enough to require a hiatus in Batman's career. Since it rather violated the premise of the first two movies, we had the Dent Device to paper over the cracks.
For me, it is both unconvincing and unnecessary, and provides a compelling reason why the first two movies are better viewed without the third.
I don't see that as a bad thing or as violating the premise of BB and TDK. For example, you mentioned escalation. That led to various innocents including Rachel dying in TDK. That film is all about Bruce realizing he can't become that guy Joker wants him to become - "I see now what I have to become to stop men like him." And during the whole film Batman has blood put on his hands. He realizes that Batman's presence is only making things worse (escalation + death of innocents), most exemplified in the rise of, motives of, and havoc wreaked by Joker.
And where is "Joker's intended return" suggested anywhere? TDK ends with Joker captured by police to be locked up with the key thrown away. Yea, he could have been in the third film if Ledger hadn't died (e.g. escaping Arkham or Blackgate), but as far as TDK is concerned Joker is out of the picture by the film's end. The only idea of Joker returning would be extrapolated from the idea of them being eternal enemies. Nothing concrete, though.
After Dent's death and the decision to create the lie, there is no reason for Batman to remain active for reasons already discussed ad nauseum. I've explained why the bridge between TDK and TDKR flows and makes sense for me. If it doesn't work for someone else, that's fine. But I haven't seen any convincing argument why Batman should not have retired at the end of TDK other than "Batman wouldn't do that". The criticisms of the 8-year-gap and TDKR's plot also don't provide an answer to that question - what reason is there for Batman to stay active at the end of TDK?
Mob bosses - check
Joker - check
League of Shadows - check (from BB, unknown to still be at large)
Scarecrow - check (was just a pawn of Ra's)
Two-Face - dead
Other villains or major threats - practically nonexistent
Plus, Batman's continued presence would be a major distraction to preserving 'the lie'.
I just don't see the reasoning for thinking Bruce didn't retire at the end of TDK. Before we knew about the 8-year-gap and such, I thought he might have been active for a short while just to help Gordon contain some of the aftermath of TDK. This would have been with an extremely low profile to avoid being hunted by authorities. Nevertheless, after seeing TDK in 2008 I left the theater thinking that after reducing Batman's presence, Bruce would stop being Batman entirely after at most a few months and let Gordon handle the rest while keeping an eye on things from the Batcave and such. Then if we were to get a third movie, a new major threat would arise in Gotham that would require Bruce to suit up again. And that's what we got.