I see no indication Kirk didn't originally
want to become an Admiral so highly positioned that his epaulets would create back problems. He just didn't realize how dull and unsatisfactory this would be, not back then. Who knows, perhaps he is actually dissatisfied with the fact that he is still mere Rear Admiral in TMP, rather than Vice Admiral already?
As for characters leaving in general, it could be that none of them really wanted a career in Starfleet. They just signed up on the five-year mission for the excellent salary (necessary for drafting people for what is essentially aggravated suicide), and not all of them became infatuated with their uniforms in the course of the mission.
At least this could work for McCoy, who could well be a senior officer without much background in Starfleet, as his high rank could be due to his medical training seniority.