Constitution Class, why replace her with the Excelsior Class?

[Praetor]

Hey Praetor! How's that Excelsior tech manual coming?

Hey guy... The project that never ends, my friend.

 

[Ar-Pharazon]

^^ Arguably, the most parsed line of dialog in Trek history.
Twelve or thirteen? What does "like it" mean? How big a "fleet" is he referring to?

Where does Kirk draw the limit, though? Would the slightly different Constellation or Defiant be one of those dozen, or something else altogether? Each a member of another dozen, perhaps?

Kirk's words can support the idea that ships looking roughly like his own ride disappeared quickly because there were so few of them to being with. But they don't have to be taken that way if some other model is more to our liking.

Timo Saloniemi

Or there are only twelve others left?

The Enterprise was an old ship by the time Kirk got her, it could be only a handful were kept in service by then.

If the AMT model kit is any indication of their intentions, the 1970's issue had 14 names in there. The later reissue had 13 (leaving out Kongo)

That doesn't preclude any new build "old style" or new build "refit style" ships. Many of us think the E-A was a new build, what with all the glitches it had.