This nicely makes the point. Jodie Foster and others have the saturation of smaller movies. In fact, superhero movies are outnumbered about a hundred to one. It's just that they tend to be more popular than most of the other movies.
We can talk about westerns and action movies and the superhero genre but they are all Action movies in a way. For a time, they were focused on a certain time and place, a mythical version of the Old West exemplified by John Wayne, Tom Mix, Gene Autry and Clint Eastwood among many other people. The westerns clearly had sub-genre shifts even within their genre from the singing cowboys to the Duke to the Spahgetti westerns to the more authentic westerns exemplified by "Unforgiven" though that was way after the genre had faded.
But the Western genre only faded in the sense that it dropped the specific time and place and instead became the Action movies of Arnold, Norris, Seagal, Diesal and so on as time passed.
The Action movie genre is still alive and yet it has also evolved into the Superhero action movie which is the most versatile of all of the manifestations of the action movie so far. I think we may eventually see fewer superhero movies per year but I don't think it's going to vanish anytime soon.
Also, I can see the first X-Men movie or, far better, the first Spider-Man movie in 2002, as the true glorious rebirth of the superhero genre.