Marvel movies are a symptom of, or reaction to, old school cinema's demise, not the cause.
Many people now are finding their adult 'movie' experiences in TV. I know I am. I'd much rather spend 10hrs in 1hr chunks over a few weeks exploring character-driven plots with superb, realistic and multi-layered dialogue, than spend 100 minutes on a one shot, edited to within an inch of its life movie. With such incredible TV shows now available some movies feel undercooked, the dialogue feels unrealistic and strange, the finales feel undeserved.
At the same time, we also now have a lot more access to gritty and real life content outside of cinema. We are bombarded with constant and free news, social media and videos online, and it's exhausting. People want to escape from that shit, not wallow in it. People want fantastical experiences at the cinema.
Also, traditional cinema serves a certain demographic very well whilst Marvel films are more inclusive. I don't mean in terms of representing different types of people in their films - although Marcel do this pretty well - but in terms of who goes to see them. They don't talk down to their audiences, anyone can watch a Marvel film and understand it. It's got broad appeal - your class, upbringing or education doesn't matter.
I'm not saying there's not a place for old school movies, but it's not the place it used to be. Movies used to be the only place for major spectacle, incredible acting, the funniest scenes, the best of the best dialogue, showcasing some of the most interesting and complex ideas. Now it's just another place to see those things, and it's an expensive one as well.
Marvel movies work in cinemas because they don't try to do too much, they aren't competing with TV, social media and streaming, they are short, sharp, fantastical bursts of fun for everyone that don't require too much thought. Often that last one's used to denigrate the films, but to me it's a big part of their draw.