They're fighting a few things. For one, fatigue. They're still making money, and probably still will for a while, but the excitement level across the board for a new movie isn't as high as when it was novel to see a new character introduced to the MCU. Building to another Thanos is the smart move, and I think Kang can be that (and I don't think he'd be defeated in one movie for two reasons: 1. He's the king of time-lost minions and indirect fighting, including of the main characters, and 2. He's also the king of time-lost doubles and alternate versions so good luck defeating the "right" one, or ever being confident you did).
I'd say I may be the only living male on the planet not excited to hear about a possible Henry Cavill casting but if they don't **** that up Doctor Doom should be better than Thanos. So as far as universe-spanning threats, they're good for a bit. And that's before you get the X-Men established in the universe and have the Phoenix force to contend with, or the Fantastic Four (assuming coming in with Doom) to add Galactus/Surfer. So Thanos type story? I'm confident they'll be there, if fatigue doesn't kill the genre.
Another thing is they're throwing everything against the wall to see what sticks, but the real question is will anything that "sticks" be as interesting as RDJr as Iron Man? Maybe, but if fans are expected to get excited for an "Iron Man" who's not RDJr in an unknown actress as Ironheart (I'd say the smarter play is leaning on the vet Cheadle as Iron Man or even War Machine, who has shown he has massive range and can do the GotG comedy bits) or anyone else in the Black Panther role it might be too big an ask.
We also have the issue of self-parody, where this Guardians formula has gotten so stale they're kicking it up a notch and turning fans off as happened in Thor 4. There's nothing wrong with comedy, but there has to be balance (obligatory Thanos meme here).
While I'll say I've been disappointed with a lot of stuff that's come out lately I did really like Loki, Wandavision, What If?, Doctor Strange 2 (though I seem to be in the minority), and the Spider-Man movies. Can't like everything, or be interested in checking out everything even as a fan. I was watching the Werewolf by Night special the other day at my friend's insistence, and while it was OK/meh it occurred to me there was a time I would have been over the moon to see Man-Thing. Now, it's more of a "hope they don't treat this like a joke" feeling and his mere appearance doesn't move the needle at all.
On a positive note, if the thing I hear about introducing the X-Men individually in specials like Werewolf by Night is true I think it may be their smartest move yet. I don't think we need Wolverine, but getting people to love a Nightcrawler/Kitty Pryde/Storm/Gambit/etc. would definitely be helped with a spotlight on their stories and powers, and not just come in to non-comic audiences as just the latest in a line of Inhumans/Eternals/etc. There are the other movies, but that's a mixed bag at best.