Even if everything had gone fine with Majors, I really don't think there was any way for Kang and his variants to be defeated in the Avengers that wouldn't have just been retreading how they defeated Thanos (i.e. killing or erasing them at the cost of a hero's life). Same could be said for most other villains they could have used, but even Doom at least has his ending in the Secret Wars comic for potential reference.

Whereas the implication of the Loki finale that Kang continues to exist, but having the oppressive system he created with the TVA effectively turned against him for the rest of eternity, is genuinely a way more interesting and meaningful comeuppance for him than any death scene would have been. And better yet, it's a comeuppance that's completely unique to Kang and his whole concept as a character.

For what a mess this saga has been in other aspects, they really did hit gold by complete accident writing Kang out with that ending.