I think 'Back In The USSR' was the straw that broke the camel's back...albeit temporarily.
George Harrison had similar problems around the same time, 'Hey Jude' was one example (hinted at in the famous 'Let It Be' row, if you listen).
However, for all that, it has seemed to me that Paul McCartney was the one of the main writers who most wanted the band to stay together. I thought that bit of film in that Scorcese documentary on George was very telling- I think it was around the mid 70s and they seem to be dissolving the band, and he really looks saddened by it.
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