It's not just that the Beatles were overplayed
I do think that's part of our problem in seeing their music the way it was seen at the time, though.
Take Yesterday, for example. To us now, it's too familiar. It just sounds like a run-of-the-mill ballad. More of a ditty, really, and a bit bland. But what the Beatles were good at was giving their audience something that was understandable enough that it would be given a hearing, but with unexpected elements so that it sounded in some way surprising, catching the listener off guard.
It's difficult for us to hear the unusual in it now, but, actually, Yesterday, despite the melodic, unthreatening tune, would have sounded a bit odd at the time. Take just the verse-length. The verse melody resolves after 7 bars, instead of one of the more usual even divisions (8, 12, 16, 32). It would have caught the listener off-guard. People would have instinctively been waiting for another bar, but none arrived. This is the kind of thing that would have made them memorable and exciting at the time. (The song also has some strange asymmetric phrasing, and though it starts in a major key, veers off into a minor feel very quickly).
It's not that McCartney was "untutored" and didn't know the rules for a ballad. He knew them perfectly well: he'd followed them enough. He knew you "didn't" write 7-bar verses. But he also knew when the rules could be broken to good effect.
Now, of course, this is nothing new. Once the Beatles established you could break those rule, people did it for the next 50 years. A 7-bar verse doesn't strike us as that odd now. But
it was then.