I think the best thing to compare the superhero movie craze to is the Universal horror movies of the 30's and 40's. Many similarities.
I think the biggest harbinger of the death of the genre is Iron Man 2. It was not a movie in and of itself, it was a potboiler and an advert for bigger things. Iron Man 2 is not a complete movie, and audiences will not be happy with a lot of such things. Harry Potter and the Deathly Hallows Part 1, sure, that's not a complete story either, but the audience is very clear which movie follows and where it goes. Mainstream audiences are not aware of Thor or the Avengers. They just see an Iron Man movie that is full of promises of what's to come, but they don't know if it's for Iron Man 3 or what it is. Not clever.
Anyway, the superhero genre will fade sooner or later. Everything does. Universal horror movies, westerns, disaster movies....all were huge for years, and then fell from fashion. What intrests me is which franchises can survive anyway. I think we can all agree that Batman, like James Bond, will be an eternal cinema fixture whether superheroes are popular or not. And Superman is always on TV or the cinema screen, and always will be. But what about Spider-Man? Or the X-Men? Why was the Hulk so popular on TV but struck out twice at the cinema? It'll be intresting to see.