Fa/tg/uy here, and appreciator of fictional worlds, magic systems, and internally consistent worlds.
One of the major ways that Marvel has especially fucked up is in the world building department. During the creation of the X-Men, they served as a handy allegory for the Civil Rights movement. As the times progressed, it became a less coherent metaphor, so the X-Men kind of became a metaphor for adolescence and puberty.
The people who read and enjoyed these comics the most enjoyed them for their deep character moments, the surprising interactions, the exploration of the rules of the setting. Like, why are the Cosmic entities of the Marvel universe so interesting? They're just bland God stand-ins, right? Wrong. They represent and explain the forces at work which hold the whole setting together.
Modern Marvel Comics, however, is still intent on getting back into the social commentary racket. Their setting is not one of superpowers and cosmic entities.. Not all the time. Instead, it's just the real world, our world, with superheroes dropped in. Everything about Marvel's version of Earth is near-identical to our world. All the same geo-political problems exist. All the same racial tensions. All the same celebrities. All the same social media shit and hashtag activism.
Marvel builds worlds the way talentless middle-aged women write smutty young-adult novels.
Mutants, Inhumans, Meta-humans, Super-science, hidden hyper advanced cities, aliens, time travelers… How the fuck can an Earth with all of those things end up exactly the fucking same as our planet? It can't. There wouldn't be a religious war in the middle east because any one of hundreds of super heroes or villains would show up and end that in an afternoon because there are more important things happening.
Instead of exploring a setting with absolutely insane potential for interesting, character-driven stories, and delving into how all these wildly chaotic elements work together and interact and conflict, Marvel took the easiest possible route and ended up with some of the shittiest comics ever put to print.