I think it's safe to say that in the case of the Miranda, Oberth, Excelsior, and probably a few others, ships would be continued to be built well into the 24th century.
On top of that, Starfleet seems to build a lot of its ships to last: The Constellation-class Hathaway was 80 years old. The Miranda class Lantree was at least that old herself, judging from her registry and fact that she was listed on Operation Retrieve in ST6. (By contrast, the carrier Enterprise, just retired, is fifty years old, older than any ship in the US navy except the Constitution, and was originally designed for a service only half that that).
In other words, I don't find anything wrong with vessels of these classes with higher designations, most of the time. (One exception would be the Melbourne in "Emissary", with his 6xxxx registry, which is more suited to the Nebula-class it was originally supposed to be).
And I think you gotta cut them a LITTLE slack in terms of reusing models. They're pretty darn expensive and time-consuming to make (money and time being two things you never
have enough of on a TV series). Ideally, for example, I would've liked a new design for the Bozeman, but this was a class specifically said to be retired 80 years ago. How much mileage would they really have gotten out of it on TNG or DS9?
No, the refit Constitution was never used on any of the shows (apart from a kitbash in the Wolf 359 scene that few people would spot on their own), but I wonder if it wasn't simply a case of not wanting to use the "hero ship" from the movies on the show (just as we never saw any Sovereigns fighting in any Dominion War battle scenes).
Moreover, I think they were pretty good at introducing some new designs in later years. Besides our hero ships (Defiant, Voyager, and the big "E"), we got the various First Contact ships (especially the Akira), the Centaur, the Prometheus, the Dauntless (sort of), and the Equinox.