But then did TUC imply the crew was getting decomissioned or the ship ? If the latter, that's just 5 years for a brand new ship.
That depends on if you accept the Okuda Chronology's dates, which has ST II-V happening over the course of 3 years (2285-87) when internal evidence from the movies show them taking place over the course of a few months at the most. They also have TWOK 18 years after The Space Seed rather than 15 (and yes, I am aware of the reasons why; and no, they can't be 15 Ceti Alphan years, since Kirk independently confirms the 15 year time-frame)
If you use the date references from the films (15 years after Space Seed in TWOK; McCoy's been assigned to Enterprise for 27 years) its a gap of closer to 10 years between V and VI.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy