I don't agree that superheroes are a genre. I think they're a concept that spans many genres. We've had superhero films that have been in the gritty crime genre (The Dark Knight), the high fantasy genre (Thor), the space-opera genre (Green Lantern), the historical/political thriller genre (X-Men First Class), the comedy genre (Kick-Ass and The Green Hornet), and now even the WWII genre with Captain America. I think that's what will ensure the longevity of superhero pictures -- the fact that they aren't just one category of story, but more of a metagenre. As long as filmmakers continue to broaden the diversity of the superhero stories they tell, as long as they keep growing beyond the stock origin-story movie, there's no reason these stories have to wear out their welcome.