I thought about it yesterday evening (picture Winnie the Pooh pondering really hard about Marvel content), and I realized the problem: I no longer see any conflict in Marvel products. In Pokémon Concierge, there is a problem where a pikachu and their trainer are not getting along, and I feel genuine sympathy for the pikachu that is not being understood by their master (pikachu just wants to ride a tube with you, you fool!). I do not know how they are going to fix this emotional problem, and when it is resolved, I feel satisfaction in them arriving at a conclusion that makes pikachu happy. 90% of Marvel content I can immediately recall has a conflict that is "x needs to be stopped or the world is going to end!", and, shocker, this problem can eventually be out-punched (or out-raced, I guess). And, like, that's cool... but I am not all that convinced the world is ever in danger at this point. Doubly so when they make a point of the world not even being a "relevant" world at all. I genuinely cared about the fate of Iron Man's daughter (Morgan!) in Endgame. I could not tell you the name of a single Eternal (okay, I'm lying, but only because I remember Ikaris and Sersi from the comics).
I don't think this is right. Not that your evaluation is wrong for you, but that apathy can't just be because there's no conflict anymore.
Eternals had genuine conflict of the not-Justice League all having completely different morals and world views - so much so that they ended up in direct opposition to one another.
In No Way Home, there was genuine conflict arising from Peter having to really struggle with what it means to be a hero, and the personal sacrifices he'd have to make to be one.
In HawkEye, there was genuine conflict arising from the fact that Hawkeye is living with survivor's guilt and that he never fully recovered from his crisis of faith stemming from breakdown after The Snap.
Ms Marvel, Kamala struggling to thrive while being divided between worlds (her conservative family vs liberalized western society; being a regular kid vs having super powers).
Moon Night, dude is literally fighting himself.
There's a lot that happens in these films/shows, and I could go on for a while. The problem I don't think is a lack of conflict. Maybe how sanitized and Disney-perfect each conflict wraps up in? But that didn't stop everyone from liking what was essentially the exact same formula for Phase 1-3.
What I think is more likely, is not just fatigue, but people's Monkey Spheres not being broad enough to enjoy all of this anymore. Phase 1-3, the pantheon of The Avengers was still relatively small compared to now. You had your handful of main heroes, each with a handful of supporting characters. It was a lot but not overwhelming. Now, there's just A LOT. It's the same problem comics have. Where the DC or Marvel Unvierses are huge, rife with hundreds/thousands of heroes. All trying to split the finite attention span of a finite readership. And even though you've got a thousand different, unique, interesting (on paper) heroes to pick from, the best selling comic books on a regular basis are still going to be Batman, X-Men, Spider-Man, etc. Your perennial, evergreen favorites.
Individuals contain multitudes, but people as a group are simple. And we tend to latch onto on a limited number of things because we haven't evolved all that much from our ancestors whose comprehension of the world around them could never really handle the abstract very well. One loss is a tragedy, a million losses are a statistic; etc.
There's just too much Marvel stuff now, divided amongst too may ancillary characters for people to care like they once did. I'd guarantee if Marvel announced an Iron Man 4 and brought DRJ back, that movie could be dogshit and people would line up around the block to see it.
Tangental to this idea, is also the fact that people are uneased and threatened by change. Something genuinely new and refreshing is nice. But once you introduce things that threaten to change something they already enjoy, there is always going to be a large segment of the population that will reflexively reject that change, based on no actual merits or demerits. Just because they're subconsciously uncomfortable now and can't handle that cognitive dissonance. This happens with fandoms A LOT. I see it happen literally every fandom I've ever observed or interacted with. I've even experienced it myself a lot in the past as well. It really doesn't matter how good something new is based on its own merits, if it's different enough from what makes people feel comfortable and safe. MCU is facing this now. I bet if you gave Kevin Feige a mulligan, he would have made the Phase 4/5 transition away from the original core Avengers a lot less abrupt feeling, because everyone kinda just retiring all at once in order to get immediately replaced by a younger generation is low key really interesting and commendable in its daring, but clearly hasn't worked out in the court of public opinion.