First off there are not alot of other comic book properties outside of Marvel and DC because both had a stronghold on the comic book industry for so long that they acquired anyone trying to compete (mostly DC) or patented so many things that it created barriers to entry (ex. Only marvel and DC can use the term superhero in their content) so this isn't a really feasible option, heck even my hero academia has to call their "superheroes" pro heroes.
Second, to clarify there is a difference between Marvel Studios having control and Disney having control. If Marvel Studios was still it's own entity that didn't fall under a parent organization like Disney, they would be free to make movies with various studios which would create a healthy market full of competition across various studios, however since ultimately Disney owns Marvel that is the problem because then they are bound to a single studio and pretty much monopolize the market.
So to be clear there is not a problem with Marvel owning the content it's a problem with Disney owning it. To add an additional point Fox created very mature marvel stories that have a different tone (which I think most can appreciate from a creative standpoint) whereas most if not all of the films under Disney have the same consistent tone. While it's not necessarily a bad thing to have a consistent tone but it doesn't allow a film to truly separate itself among the rest and stand on its own. The Harry Potter series is a perfect example of a series of films where each film can stand on its own and be memorable. Most like Marvel because they have done a good job of creating 22 sequels for the most part however most of their films are forgettable (except Iron Man) and will most likely be seen as a generational fad to future generations. The first spiderman series, first x-men, and Logan are memorable and don't make you feel like you're watching the same thing. Unfortunately with Marvel being under Disney they have inherited the Disney brand which means that if they try to take their characters in different directions that threaten that brand best believe that daddy will slap their hand and tell them NO. This is unfortunate because we have seen that under other studios that Marvel has worked with that they are willing and capable of taking their characters and storylines into different directions.
For true fans of superhero content most want the cinematic versions to truly encompass and represent the characters that they liked in the comics and sometimes that doesn't fit into Disney's cookie cut brand. Let's not forget that characters were drinking, doing drugs, and having sex in the marvel comics and I doubt we'll be seeing those representations anytime soon