I'm trying to pick a plausable POD. Ringo wouldn't quit, not that early (actually it was 1964, not 1966).
Nicol was the replacement for an Australian tour when Ringo had a tonsillectomy. Maybe the surgery goes south and he's forced to quit/dies on the table. Something shocking like that.
The tour is called off momentarily, but then continues as a tribute to their old friend, with Nicol in place since he was there already.
Yes, Nicol was a bit of a jerk (apparently), and he clashes with the rest of the boys. I don't see him staying permanently (or till 1970). Maybe he leaves after a year or so. Session drummers thereafter, and Beatle friends drumming on tour. OMG PETE BEST???!!
After 66, maybe they don't need a permanent drummer since they aren't touring anymore? Paul moves to drums, they bring in pal Klaus Voorman to handle the bass?
Anyhow, I don't see Nicol lasting long as a Beatle.