McCoy also makes the claim of having been the ship's CMO for what, 26 years? Does that mean he was still considered to have had that role after he had resigned in the years between TOS and TMP? (I know, it is like the Meyer films don't even acknowledge TMP happened. They seem to think Kirk never saved Earth till TVH or somesuch. But even so.)
I've always figured they had another couple 5 year missions between TMP and TWOK, with Kirk stepping down or forced down toward the end of that 3rd set of 5, and then it being back to biz as usual, perhaps an openended mission, after TVH & TFF. But the 'save them for special assignment' notion does have some credibility, I'll grant you.