Disney more than likely is well aware by this. I mean, they just have to. Legally speaking, they still don't have a monopoly in the movie industry. Seeing that Disney buying Fox was the biggest thing in Hollywood since... Disney buying Lucasfilm I guess, Disney will more than likely cool itself down, at least till other companies like Warner Brothers, AT&T (if they have any other studio besides WB), Sony, Universal, or other smaller independent studios will grow big enough to pose a threat to them.
But in the comicbook superhero industry, yeah they have a monopoly. Technically speaking there are other superhero universes that they can't touch, but I doubt that the DCEU (if it even is alive at this point) will suddenly become bigger than the MCU, even tho it obviously has the potential.
Tho I recently observed how the lines between superhero movies and summer blockbuster movies becomes more and more blurry. Sure, superhero movies were initially just a branch of summer blockbuster movies, but the MCU proved us that they can be so much more than just mindless action movies with cool special effects. Nowadays every major franchise has its own comics, whether they are indies, IDWs or whatever. For example, if Paramount would suddenly decide to adapt an IDW Transformers comic as a movie, would that be classified as a comicbook (maybe even superhero) movie?