It’s pretty established that Paul was the workaholic that kept things going as long as it did but the spark was no longer there as it had been during their peak just a couple years earlier.
I think the fact that they called it quits when they did after the spark was no longer there for all but still cranking out classics helped cement their incomparable legacy. Opposite of the The Stones who were happy to keep their thing going forever and get it back out on the road from time to time.