1) Agreed that Kang could be recast with minimal to no explanation. But though I haven't been watching Loki S2 yet, I'm not yet sold on the character and his motivations - whole universes going to war with each other is a very shapeless and psychologically incoherent concept. Thanos' universal culling, on the other hand, involved death on an unimaginable scale, but it also made tangible, intuitive sense, and directly related to real-world environmental concerns.
2) The overall core problem here is shaky storytelling, and bad writing. Quantumania made Janet look foolish for not warning her family to never attempt to contact the Quantum Realm, because she knew a dire threat lurked there, gave Hope nothing to do, and wimped out on Scott sacrificing himself (at the very least temporarily) to keep Kang contained. Black Widow and Eternals were pointless stories at best - if Natasha had a whole squad of Widows to help rehabilitate (or half of them, at least), why did she tell Steve the Avengers were her only family/responsibility in Endgame? Will Bruce's alien son be an actual character, or will he be a forgotten, weird joke? How do our African-American heroes feel about Wakanda? Can't Pepper extend Sam a home loan for his boat, or can't he profit off his celebrity even a teensy bit? If a teen at MIT can build an Iron Man suit with help from YouTube, why can't the government? Etc.
On a broader scale, what nincompoop thought a Secret Invasion series without the major heroes was a workable idea? (If it was just a matter of desperately needing content, why not more Agent Carter, or a Maria Hill solo outing?) Where's our Shang-Chi follow-up? And who the heck asked for an Echo show? It's one thing to make an Agatha show, set safely and cozily in centuries past, but the more Kingpin is around, the more people are going to want to see Spidey.
(And weak writing isn't just a Marvel problem, it's a Disney problem. Boba Fett, Mando S3, Ahsoka, and Dial of Destiny had major script problems, and I'm guessing Willow did, also. Furthermore, Pixar has been shaky, Strange World and Haunted Mansion bombed, and they've nearly run out of live-action remakes, to say nothing of their failure to build up/maintain other franchises, including Pirates and The Muppets.)
3) Sounds to me like the main person problem is Bob Chapek and the other top Disney execs, who demanded way too much series content from Marvel and Star Wars in a rush to build up Disney+ as soon as possible, with the added mistaken short-term thinking that series would remain more important than movies after the pandemic subsided. I'm not saying Feige can do (or has done) no wrong, but I'd be astounded if he himself didn't think he was being stretched far too thinly. An interconnected, internally cohesive universe needs a strong guiding hand, as the success of the MCU itself amply proved.
