A "recommission an existing ship as 1701A" does also makes sense for why Enterprise-A was "retired" (along with it's crew) a few years later after STVI (about 5 according to the official timeline).
Yorktown or not, a ship undergoing recent refit or upgrade could be ready much quicker than a new build, and then be used for a few years and still reach it's end of life after STVI.
Then again, we don't know enough about Fleet Ops, build schedules, phase out of equipment types - there might be plenty of reasons to replace the
Constitution+ ships with the
Excelsior types. I know! Maybe the Connies were Windows XXP and MS had discontinued support! We know after the disastrous MacOSX/
Excelsior interface failure in STIII that they switched them to something else. Maybe Linux
Don't give the credit to FJ, Q, you did a great job back there squaring ship names to actual canon appearances.