I'm not sure it's burnout as much as it's just difficult to shift gears from huge climactic story to mostly a new run of origin stories.
In the comics you would often get kind of a breather issue where you got to see the characters reflect on the new (old) status quo and then hook you on the last page with the emergence of a new conflict.
You can't have a whole movie (or series of movies) dedicated to catching your breath. The medium just isn't suited for it.
I actually think this is where the TV shows make sense. They provide the opportunity for a smaller-scale, lower-stakes, more contained story, but one that can be told over multiple hours. But it's worth noting that the films aren't exactly "catching their breath." I mean, in terms of the multi-film-long arcs, they are. But they're still individual stories with action and stakes involved in each film. They just don't feel as connected or as if they're building towards anything (well, up until the new Ant-Man film they hadn't).
The difficulty, I think, is that people expect stakes like Thanos, and they just aren't getting it. Their memories are short, and they can't put themselves back to, say, 2009/2010 when the biggest thing we were building towards was "We're gonna make a team." Now we're kind of back in that same state, and audiences just don't care. For that matter, they don't even see the "team" forming.
I was definitely ready for a new movie after the COVID delays subsided, but I don't think they did a good enough job getting people invested in the new slate of heroes. They really kneecapped themselves in a way, by giving Cap and Tony their perfect exits it precluded them from having those characters around in the background to bolster the new roster.
Yeah, but I think that it worked better for their stories and offered a chance at a real exit for them. But right now it does feel like Endgame was kind of the end of the MCU, and what we're seeing afterwards is almost a new universe in the same setting, if that makes sense. There isn't the kind of continuity between the old and the new, and as a result, people just aren't as invested in these characters.
I had exactly the opposite experience. I read comics religiously from 91-'07 and when I stopped it was because the titles I was reading hadn't grown with me.
Not to say they were without value, every comic is someone's first, but they felt like rehashes of the same stories I'd been reading for over a decade. I couldn't fault them for being the same thing they always were, but my own tastes were changing.
For me, the experience with comics was probably similar but set in different decades. I went from buying issues at the drug store here and there in the 80s to hitting the comic shop in the late 80s/early 90s and discovering titles that I really wanted to collect, to totally burning out with my first big "title shift." I was into the X-men back in the 80s/early 90s, and had pretty much ignored most other titles just for them. At that point, there were basically three main books: The Uncanny X-Men, X-Factor, and New Mutants. I really only paid attention to the first two, and then mostly to just the first one. There were other books like Wolverine and Alpha Flight, but they mostly kept to themselves and didn't interact with each other.
And then 1991 hit.
That was the point at which they exploded into, like, 6 different titles, each publishing monthly, each with storylines carrying over between the titles. You had X-men, The Uncanny X-Men, X-Factor, New Mutants, Excalibur, Wolverine, Alpha Flight, etc., etc., and right with the first storyline, I just...dropped off. I couldn't afford it. I was a kid with an allowance that let me buy maybe 1-2 comics a month, and I couldn't afford to be buying 31 different titles just to figure out what the hell was happening in the story.
That was also the point at which I discovered Dark Horse comics, and started collecting the old Classic Star Wars series, Knights of the Old Republic, and some others. But from that point forward, all of my comics consumption was either sporadic at best, or consisted of much later in life picking up TPBs and getting caught up on this or that storyline. And that's pretty much how I interact with them now. I'll grab a TPB once in a while and read it, but I don't collect (except for the stuff I still have from the old days, most of which is in crap condition and is largely worthless).
But I think that's often how it goes. Either you can't afford the habit, or you just age out of it. You stick with a story for a while, with a group of characters, or whathaveyou, and then after about 3-5 years, they switch creative teams (new writers, new artists), and you realize that it just doesn't give you what you want anymore, and you move on.