I think drugs screwed up John Lennon's sense of perspective - it made him, in a way, insane, the way he brought Yoko Ono into the studio like that, and needing her by his side even when he went to the toilet. That's not the act of a sane individual. John's drug problem really was the problem for the group.
As much as I would have liked to have seen them perform together as a group into the seventies, it was probably for the best that they quit when they did. As the old saying goes, get out while the getting's good! They left while they were still at the top of the game, and a lot of other groups don't have enough sense to do that. Or the courage. Of course, the Beatles didn't quit because of courage - they quit because John was leaving the group. If John wasn't leaving, George might have been persuaded to stay on, since George Martin and Geoff Emerick (and everybody else) decided they'd been wrong about George after hearing his work on Abbey Road, and were going to give George a wider berth to work with, and more assistance and attention. That would have satisfied George, and kept Paul McCartney at bay and behaved (he was notorious for rushing George's work so he could continue on with his own).
We are lucky to have had them for the 10 years they were around, for they really were perfect in what they put onto their albums. WE STILL ENJOY THEIR MUSIC! And not just one or two songs, but practically ALL of it. That's some testament. And that's something none of them could produced on their own. Not to that extent.
