Obviously, what I'm about to say can't apply to everyone with that complaint, because some people just have different tastes. But I have a hunch that at least for a lot of critics (Baby boomers and Gen X'ers), the reason for this is simply because comics were a medium for children when they were growing up. It's something they remember from their childhood and they look to comic book films to adhere to that sense of innocence and nostalgia. Even though Batman has roots as a darker character, the Batman they grew up reading was nowhere near the hardcore and cynical Batman of the 80s. But, seeing as I was born in the 80s...that's the Batman I've always been accustomed to. If I were to sum it up, I think Nolan's Bat-films were a great blend of 70s and 80s Batman. There was darkness, there was cynicism, there were truly scary villains and weighty themes, but at the same time Batman remained a truly heroic figure with altruistic intentions. And I also want to just point out that despite the Schumacher years, Burton's Batman did wonders to change the mainstream palette when it comes to Batman. Burton set the precedent for a darker version of the character in the movies, then Nolan took that precedent and ran with it. There needed to be that progression, I feel. I don't know if the mainstream would have been ready for something like The Dark Knight in 1989.
There probably are a lot of the older generations who do feel that way but based on my own personal experience, the people that often made such statements were often not that old compared to me (I'm an early 90's kid btw). Most of them grew up during the 80's and 90's. Once again though, there is absolutely nothing wrong with not liking TDK because of its serious nature and wanting something more lighthearted. My main problem is when people try to pass their personal taste in Batman as an objective criticism towards the Nolan films. I've never personally cared for the campy style of the Adam West show but I still respect the show for what it is and understand that it is purely a taste thing for me (though to be fair to both sides, not everyone does).
I don't feel the Nolan films can really be summed up to a specific era. The way I always summed them up is by their main components. I think there are 3 major components from the comics that contributed to making the Nolan films what they are:
1) The realistic crime drama setting and feel of comics in the late 70's, 80's, and early 90's
2) The Year One/Early-Career Batman stories.
3) The complex political themes found in Frank Miller's books (specifically in The Dark Knight Returns)
Even with all this attention on Superman at the moment, I don't think he'll ever fully reclaim "the throne", so to speak. Yes, he will see a healthy spike in popularity, which is awesome...and he should get the attention he deserves.
Sure, Supes came first and paved the way, but Batman (as a character through the years) perfected what it means to be a hero. He's infinitely more interesting, IMO, both visually and psychologically. And, to boot, his rogues gallery ****s on Superman's. So yeah, I'm happy Supes is back in the spotlight...but he only has a few more years to "reign" until the Batman makes his triumphant return.
To an extent, I do agree with this. It will be extremely difficult - dare I say near impossible - for any superhero to take the throne from Batman, assuming that the quality of his products doesn't slip up.
Batman has one advantage over all superheroes, which is his universe. The Batman side of the DC universe is so fantastic that even if one was to hate Batman as a character, he or she would still tune in to Batman stories due to everything else - the villains, the supporting cast, Gotham as a whole, etc.
His fantastic rogues' gallery alone is able to attract so many people and it is no coincidence that a lot of the Batman films we've had up until now were mainly marketed on the villains. I'm sure you've heard the saying "A hero is only as interesting as his villains".
This has created an almost invincible barrier for Batman that no other superhero can break. Even Spider-Man, who in my opinion has the best pocket universe right after Batman's, still doesn't have a universe on the same level.