I was watching The Mark of Gideon (TOS), and to my surprise, I think the reason there are no
Constitution-class ships in TNG is revealed!
The design was compromised, someone else had it.
The Gideon-planet copy is so similar that Kirk does not realize that it is a fake for several hours. For him that is essentially
having an imposter pose as his girlfriend and not noticing for several hours!
A number of nearby episodes in Star Date order deal with capture of the Enterprise ("Day of the Dove", "The Time Trap",), and that was a major plot point of "The Enterprise Incident," not that much earlier. Klingons have been aboard and seen its tech. Romulans have been trying to study it for years, and maybe stole an early Star Ship design to make the BoP. And now, some random planet has an exact copy, or a captured lost cruiser rebuilt to look like the
Enterprise.
The admiral tells Spock not to beam down and start a war. A war with the constrained local population, or a war with another threatening power who wanted Gideon to be part of their government?
It would be major egg-on-the-face of Starfleet to retire the
Constitution right away, so they refitted it, changed it enough for security until its replacement, the
Excelsior, could become a reality.
The Miranda would be newer then, and stays around because it is different and re-configurable enough to keep it out of enemy hands. I also am one of the fans who favors imagining that many of the background
Mirandas we see in DS9 are really
Centaurs.
Oh yeah, and...
WHEN ADMIRAL MORROW SAYS THE
ENTERPRISE IS 20 YEARS OLD, IT HAS BEEN JUST OVER 15 YEARS SINCE THIS EPISODE; HE'S NOT REFERRING TO THE REFIT, HE MEANS IT HAS BEEN ABOUT 20 YEARS SINCE THE DESIGN WAS COMPROMISED. HIS LINE FINALLY MAKES SENSE!!
I love it when a lesser-known episode can take on some greater significance that makes it seem more important