One guess is that when Starfleet was going to retire the Enterprise after she returned from her training cruise (Battle with Khan). they were preparing to build or were building a new Excelsior-class Enterprise to replace her. However with Excelsior's failed pursuit and refit prior to Captain Sulu gaining command, the new Enterprise might have been delayed until they could work out the bugs on Excelsior. Then they added new systems that increased the time to get the new ship completed. Newly demoted Captain Kirk, who while a major hero, was probably still in trouble with the Starfleet brass, got himself a renumbered Constitution-class starship for about seven years.
Enterprise under Kirk is recalled to be decommissioned. Kirk and his crew seem to know they are due stand down within six months at the start of the movie. It isn't clear in the film why she'd being decommissioned and why Kirk and his staff were likely retiring within the year, the logical explanation is that the new Excelsior-class starship Enterprise was expected to be finished by then. Kirk's command staff is getting older and may not feel like getting acquainted with the new Excelsior-class after decades of training and retraining on Constitutions.
I seem to recall someone having the idea that the next Enterprise would have been NCC-2001 had they not had the extra Constitution-class ship added to the list prior to the Excelsior's completion of her trials. Starting the trend of -A, -B and so on. If the older trend had continued, the Galaxy-class Enterprise could have potentally been NCC-70101 or something of that sort. (NCC-70701 also possible)
Manditory retirement age use to be 75 years old based on the "Counter-clock Incident" involving Robert April. Kirk was several years away from being forced to retire by age.
