I always figured that 1) Enterprise was one of the few ships that completed the 5 years without losing ship and/or crew (a dwindling fleet could have launched the refit) and that 2) Any other commanders that did this were not on Earth at the moment, and out on other missions. Kirk was fresh off the mission, whereas most of the Earthbound deskbound Admirals were years away from their active service. Just Kirk using whatever he had to get back out there.
I do like the idea of random ending, not a scheduled wrap up, but in my head canon, thats what had to have happened at the end of the TMP era mission - but the concept works just fine for the end of TOS. I have always entertained the notion that something big happened... something that sparked the militarization of Star Fleet, the change in uniforms, the newly amped up aggressions with the Klingons, and the benching of Kirk/retirement of Enterprise - (and I would like to think it was years into the Refit Mission, and NOT just V'ger...)