The Beatles quit at the perfect time. The Law of Diminishing Returns was setting in big-time on them.
Die young and leave a handsome corpse. Was that Jim Morrison? He certainly avoided the diminishing return law.
I don't see that at all. Their last two albums, Let It Be and Abbey Road , were both really good. Given the immediate successes of the individual careers of Paul, George, and Ringo, it didn't look like the Beatles were slowing down much. It could be argued that McCartney and Wings became the biggest act in the world once the Beatles broke up.
It was clear that the personal relationships were strained (particularly between Paul and John) but if they had acted like professionals and set aside those differences, they still had a lot of creative ideas. It was also clear that the world had not tired of the Beatles yet.