I've been going through such a phase recently about thinking of the various fandoms I've been a part of and the reasons I left them, maybe this line of thought is mainly due to the countless videos I've been watching about the new Spider-Man movie coming out and my absolute disinterest in watching it. (One of my brothers has also been bugging me about it every time I see him 🙄)
Some of my depart from the MCU fandom (I was never really a DC fan) is just in general a superhero movie fatigue at this point and how formula based the movies are now. But also it comes from some of my own interactions with some fans on my own fanfics that soured me a bit to the enjoyment of the fandom, but I think I could have put it aside if I wasn't also at that point just done with the entire franchise.
Because for me personally the entire MCU had been built around Iron Man and Tony Stark. He was the superhero that launched the entire MCU and most, if not all, superheros that came after him interacted with him or were influenced by him in some way (positively or negatively). So when he died in Endgame, that was kinda it for me. And there have been only two Marvel movies that I've seen since watching Endgame in the theaters, No Way Home and Multiverse of Madness, and at this point I have no interest in seeing anything else.
All these movies that are coming out now are catered to an audience that I have never been a part of, comic book fans, and I love that for them. I love seeing their reactions to these movies and pointing to the comic books that are being referenced and giving their opinions, both negative and positive, about the movie but I'm not that fan. I didn't read comic books as a kid, I did watch the old X-Men and Spider-Man cartoons but it didn't impact my life like it has for some other fans. So for me, my introduction into these characters was from the movies and the first superhero that I connected with and followed his journey throughout the movies was Tony Stark.
Robert Downey Jr.'s Tony Stark was my superhero. He was the one that I got excited to see on the screen or get referenced in the other movies and read a lot of fanfics about. So when he died I couldn't even read fix-it fics about it, I grieved that character like I would loosing a long-term friend, I bailed my eyes out in the theater at his death and after that I was done with the MCU.
I think I was so affected by his death because he was the first character that had connected with me personally, as a movie character I've read more books than watched movies, and a character that I followed since I was a kid. From 2008 to 2019 Tony Stark was my superhero, that's 11 years of following a character's journey, so yeah after his story ended in a way that felt like it had come full circle, as tragic as it was, I was like 'yep I'm done, I don't have the energy to follow another character's arch'.