Actually, according to most sources ( all of which may be non-canon ), the Enterprise-A was not a new ship. It was the rechristened U.S.S. Yorktown, which was an old Constitution from TOS. She was refitted like the Enterprise from The Motion Picture. It was done this way because they didn't have time to build a new ship to give one to Kirk at the end of Voyage Home. Also, makes some sense given that Yorktown was also crippled in ST IV and may have needed a minor refit anyway. It also may explain why the ship doesn't work so well in ST V. Most of the new systems haven't been tested and there has been no shakedown cruise for working out the bugs. So, this ship then has the same long service life as the original Enterprise and needing a decommissioning at the end of ST VI.