I've still got my theory that 'Enterprise A' was kind of a ship on permanent stand-by, whose crew were in a state of semi-retirement except for a few occasions when Starfleet assembled them together and sent them out on a special mission that required the specific talents of that
crew and that
captain. Certainly, it's clear they haven't been out in space together for some time at the start of STVI, and even the events at the beginning of STV could
be interpreted the same way. The crew are all off doing their own things and Uhura has got to "get the band back together" for the Nimbus III mission.
Behind the scenes, I assume 1701-B was already under construction in the timeframe of STIV to STVI, so Kirk's "new" Enterprise was probably only ever
intended to be a temporary stop-gap by the powers that be in Starfleet Command.