The answer of what stops this is something you are taking as given changes. I am not sure what it is, for the rise of superhero movies (at the expense of others) is the rise of easy access to movies and demographics changes (non-Americans being included). To stop this something underneath the surface and probably not even thought about much will need to change.
Note: This isn't saying anything that is actually bad, it just the way big systems that used to work fall apart. Assumptions are made because they work, then something changes and they stop working.
Note: This isn't saying anything that is actually bad, it just the way big systems that used to work fall apart. Assumptions are made because they work, then something changes and they stop working.