One other thing to take into consideration is the likelihood that ships pre-Excelsior
(for my money anyway) probably weren't designed with 100-year lifetimes.
When the original Enterprise
was retired and destroyed, it was presumably 40 years old. I'd like to think that TOS and TMP era ships were designed with a 50-year lifespan in mind, akin to modern aircraft carriers, but most (especially the Excelsior
) went beyond that. By the time of the Galaxy
, 100-year lifetimes became a design goal, maybe even with potential for longer actual lifespans?
As to shipyards - we know of at least 13
. Your 23 figure may be a good guess then.
(Nice new young Ro av, btw.