The theater experience is relatively expensive these days, from the tickets themselves to the snacks/drinks,etc. Factor that with the normalization of streaming services having original content of comparable movie budget and quality. Factor that with smart HD/4k grand sized TVs with surround sound systems, etc. And obviously the incentive of going to a movie theater will diminish.
If you're taking your girl out to the theater on a Friday night (15$ per ticket x 2, 10$ popcorn/food x 2, 10$ drinks x 2 = 70$), chances are you'll wait for an "event" movie or a movie that one of you were dying to see. For everything else, there's probably something comparable or better at home, without having to spend upwards to 70$ to be locked in for 2 hours.
So I wouldn't say Marvel is diminishing anything, they (Disney) just have the most IPs to take advantage of this shift, but Aquaman, Jurassic Park, Fast & Furious, etc.. are all film franchises that aim to do the same thing.