There've been plenty of people who wondered what was to happen to the superhero movie, once the bubble finally bursts. More often than not, the answer is that it'll go the way of the Western, that it'll gradually lose all relevance in the popular culture.
I don't know about that. I think of superhero movies in terms of the sci-fi genre. Once that box exploded, with Star Wars, it became very hard close up again, in a way that Westerns haven't really experienced since the end of their heyday, when they held a virtual monopoly on the public's imagination. Half the reason superhero movies exploded in the new millennium was because Hollywood finally figured out how to duplicate Star Wars' blockbuster model. Suddenly, there were hot new franchises all over the place, of viable endurance and massive popularity.
The problem, if anything, is to keep superheroes from becoming like horror movies. Horror movies have long had their sway with audiences. In the '80s they took on new life with a slew of franchises. The problem is, horror films became increasingly hard to translate to wide audiences, unless a true breakout occurred, which does continue to happen. The thing is, no one thinks of horror movies as dominating the box office, the way they did in the '70s, before Star Wars. Tastes change. Something will replace superhero movies as the hot new thing, but superhero movies won't go away, like Westerns have for the most part.
The problem will be in finding the balance that prevents them from appearing too insular. In this age where the massive Avengers franchise exists, this can seem like a ridiculous dilemma, because eight years and over a dozen films later, no one's arguing that it's tough to keep track of what happens in these movies, because they're designed to operate independently, with little need to know what happens in any one film. You can watch one of them and simply expect a good time.
That's it, really, the expectation that you don't need to invest too much into the experience. Anytime a blockbuster goes beyond that expectation, barriers begin to form, and opinions sharply decline in their generosity toward the film.
It's not that difficult to see how this tracks. Everyone loves Star Wars, because the original movies were filled with romantic breeziness. The second one, The Empire Strikes Back, deepened the story just enough so that fans thought it was a worthy follow-up to the original. By the time the prequels were made, however, you suddenly needed to know so much more, such as why it was at all important to care about the little boy Anakin Skywalker. It wasn't simply a matter of expectations, but having to invest in the material something beyond sheer enjoyment. In fact, under circumstances like that, sheer enjoyment becomes the last thing possible.
Where superhero movies started looking for something other than entertainment was the misleading success of The Dark Knight, which proved to be a spectacle for reasons other than Batman's adventures, but the spectacle of the unexpected performance from the late Heath Ledger, whose death alone brought attention to the movie. People loved Spider-Man as a lovable sadsack, and because watching him swing through New York City was a life-affirming exercise post-9/11. They didn't particularly want the Goth dorkiness of Spider-Man 3 or Marc Webb's deeper analysis in the Amazing Spider-Man films. Batman v Superman: Dawn of Justice has proven a powderkeg precisely because it tried to be operatic instead of just a big dumb fight between its title heroes.
These ambitious superhero movies have their place, but they'll never be as popular as the lighthearted, irreverent material that continues to dominate the box office. You try for anything else, and people will just crap all over it, because this is not the kind of stuff they want to think about. They go to the movies to escape. That's how hits are born, the ones that are still fondly recalled years later.
Hollywood has developed a new problem, in that there are so many movies with blockbuster expectations, they can't possibly all end up blockbusters, and suddenly there are more and more blockbuster box office busts. This can't at all be surprising. The more demand you make of your audience, the more careful you'll have to be to find it, because otherwise, it simply won't be there. It's the blockbuster numbing effect. What otherwise might have become a generation's defining movie memory will have been lost in the shuffle, simply because you didn't expect it would be so difficult to get lost in the story. Sometimes, it really is. The hit movies are the ones that make it look easy.