The turning point, IMO, was when John realized he couldn't continue as a "Beatle" on the same level as Paul could, by mid-1969.



Which means.....John's songwriting quality and interest in being a Beatle had waned to the point where he had to walk instead of just phoning it in like he did in Let It Be and (most of) Abbey Road. John's ego wasn't going to let him be compared to Paul's stronger A-side and album material and George's rapidly improving songwriting within the group any more, but he could start over with Yoko and be the cool, hip, avante garde Ex-Beatle.



Also, John's competitive nature didn't allow him to stick around on a sinking ship. You will recall how upset John got when Paul announced to the media that he was quitting the Beatles...when John had actually quit half a year previously. It's not unlike the argument between The Who and Jimi Hendrix at Monterey Pop over who was going to go on stage first--neither wanted to follow the other. In the same way, John wanted to quit first, instead of finding himself in a group where George or Paul walked out first. He started the group and he wanted to be the one who ended the group.



His partnership with Yoko gave him the confidence to finally jump ship. John stated that he thought about leaving the group as early as 1966 when they stopped touring, but he didn't seem confident enough then, in the midst of his acid indulgence period, to do it then. He also ruminated about the end of the group when Brian Epstein died....but that was put on hold when they planned MMT and going to India. When all that fell apart, he became involved with Yoko and heroin heavily...which probably forced the issue---he couldn't pretend any longer.



So that was the turning point...when he realized it was ending for the various reasons anyways....he wasn't contributing at his former level of excellence....and him wanting to be the one who walked first.

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