As far as the Beatles were concerned, while the energy in this thread has focused on John Lennon, it's worth remembering just how bad the period immediately following the Beatles breakup actually was for Paul McCartney. Yes, he still was able to sell records, but during that period, he had lost much of his critical reputation and even his sales were arguably on a downward slide by late 1971. After all, Imagine outsold Wild Life when they were released.
Now the luster of his Beatle years could never entirely have gone away, but if Wings had been the failure they seemed to be when they released their debut album, if he isn't able to regain his reputation to a significant extent with Live and Let Die and Band on the Run, if the subsequent Wings albums were just as poorly received as Wild Life was, McCartney could have cemented a reputation in the 1970's as a has been at least among music critics. That is, someone who once had been utterly brilliant, but had lost his original spark entirely.
Albeit, given his immense talents, it stands to reason he would have recovered from his slump eventually, and it's hard to imagine him continuing to put out material that was received as Wild Life was. But McCartney's position was surprisingly precarious between the completion of Abbey Road and 1973. Again, I think he was bound to recover eventually and reestablish himself, so this is probably horribly unlikely, because the period in question probably went as poorly as it reasonably could have.