I'm literally in the middle of watching a big bubble burst. It's kind of exciting.
"...It's possible that they might not."
That's how fads work, Bob. Superheros are big now, until... they're not anymore. Objects in motion tend to stay in motion? You assume that these franchises will remain popular. But, that's the thing, these markets change suddenly, and drastically. Suddenly the next genre defining Star Wars syle film will come out, and Hollywood will have a glut of superheroes that no one cares about anymore. These companies stand to lose a lot of money when this DOES happen, if they can't adapt. Other companies will adapt, and chase after the new fad, while mistakenly believing that Superheroes aren't profitable anymore. No one who found themselves in a bubble fad ever thought the gravy train would end. Then it did. Go look up Tulipomania.
This genre isn't self perpetuating, and it isn't permanent. It will probably last at least another decade, since we're in the middle of a golden age, but in the end it's a fad. Traditional narratives haven't died either. People will always want more then never ending genre installments with no nutritional value.
Your vision has been clouded by your bias, bob, but I will feel sympathetic when your Marvel films are replaced by football movies, or films about clowns fighting dinosaurs or something.