Enterprise was turned over to Will Decker
I like that, too. I agree that it seems a bit contrived that the core group stayed together all those years in the same jobs
Not everyone would have needed to be aboard the ship the whole time, Chapel left to become a MD, and then came back. There is conjecture that Chekov left to attend a security school, then returned.
Rand disappeared during TOS itself, returning as a transporter tech, wasn't aboard the ship during TWoK, later she was aboard the Excelsior. Uhura could have been posted to another ship for years. Scotty was there for the year and a half rebuild, but the in-between time?
Both Spock and McCoy left Starfleet altogether. Spock return because of V'ger (and would have joined Decker's crew for the mission). McCoy was pulled back in by Kirk.
I think that originally Sulu was Decker's first officer, either that or Kirk sent that individual into the bowels of the ship, or left them on Earth. Either way someone there got screwed.
How many really stayed with the ship after TMP?
Kirk was off the ship again , and maybe McCoy with him. He (and McCoy?) were aboard for the inspection cruise.
Chekov left again at some point to become the first officer of the Reliant, he wasn't officially reassigned to the Enteprise until the end of TVH.
Sulu, from his own comments, wasn't a part of the Enterprise's regular company during TWoK, he was there temporarily. He rejoined the ship at the end of TVH, and for TFF. He then became Captain of the Excelsior.
Transporter Chief Kyle never returned to the Enterprise after TOS. We saw him briefly (iirc) aboard the Reliant, hopefully he wasn't killed.
Chapel left the ship (after TWoK?) to be next seen working at Starfleet HQ.
So, the core group doesn't
seem to have been continuously assigned to the ship.