I haven't seen Peter Jackson's documentary series on the Beatles' Get Back sessions, but do find some interesting analysis and question in this review. The big question that Snowdon tackles is why they broke up when they did. If they didn't actually hate each other, if the sessions weren't all misery and boredom, then why not stay together a while more?
I suspect it was the prospect of being in the shadow of their own legend that prompted them. The idea of hearing the sentence, "They're great, but they're not quite the Beatles" applied to themselves. Since their reputation and impact in the 60s got quite a bit beyond rock stardom, settling down to mere rock stardom would have diminished them. So best to go out on top, while they were relatively still on top.
Consider the Rolling Stones. In the Beatles' absence they could finally occupy the top slot in the seventies. So they had a hot few years, then came upon a period of diminishing returns. Their last essential album is probably Exile on Main Street from 1972. Their last interesting album is probably Undercover, from ten years later than that.
So a defensible decision on the Fab Four's part.