Its just too bad they didn't get along after they broke up. John only lived, what, 9 years after they split? And it seemed he was finally, really, coming back into the limelight until what happened happened.
Their relationship was really only bad from late-'69 to mid-'72. Once the Beatles were officially dissolved in '73 and they were no longer bound together contractually or financially, their relationship rebounded. During '74, Lennon was the former Beatle who spoke most positively of a possible reunion, and he and McCartney recorded a jam session in Los Angeles together. (It's not worth listening to, honestly.) Lennon was invited to (and intended to attend) the Wings sessions in New Orleans for
Venus and Mars, but his reconciliation with Yoko scuttled that. The relationship between Lennon and McCartney cooled slightly after that, but it was never, ever as bad as it was when the band broke up and immediately thereafter. They were watching
Saturday Night Live together at the Dakota the night that Lorne Michaels offered money for a Beatles reunion -- and they were tempted to head to Rockefeller Center and take him up on it.
Lennon's worst relationship after the break-up was, curiously, with George Harrison. Lennon backing out of the Concert for Bangladesh dealt a blow to the relationship to the two men that never really healed, and at the time of Lennon's death things were not at all good between them. (Harrison's autobiography,
I Me Mine, ignored Lennon, and that angered Lennon intensely.) If the Beatles had reformed circa 1976, the line-up would probably not have had George; it would have been John, Paul, Ringo (and/or a session drummer), and sidemen like Jesse Ed Davis. It's not that unlikely; had Lennon gone to New Orleans, it likely would have happened.
