It's vague when they got up to registries in the 5xxxx range, but regardless, Oberths aren't combat vessels. They're science vessels and maybe some other light duties, like ferrying people between starbases (i.e. the Cochrane in "Emmisary"), or testing new technologies (the Pegasus) etc. There's no reason older ones couldn't have been upgraded with new computer systems and warp drive and there's no reason newer ones couldn't have been built with those upgrades.
The Ford thing's a bit of an exaggeration, but some designs may simply work really well, even after several decades. Starfleet may simply stick with designs when it finds a good one, something that clearly applies to the Excelsior.
Again, I agree that older ships can be upgraded (at least up to a point). And again, that wasn't my point. My original point in the Oberth essay was that the Oberth class is a bad design, and there's absolutely no reason why it should have survived for 80 years of continuous builds when other ships were being designed and built. Why didn't the Constitution class last that long? Why not the Constellation? Why not the Soyuz? They were contemporaries of the Oberth class, and much better designs, yet they were being decommissioned even before the start of the 24th century.
Why do some classes last longer than others? Probably a variety of reasons. Some may not be as adaptable to various mission profiles as others. Some may be more costly to built and/or maintain.
The Excelsiors, I suspect, ended up effectively replacing the Constitutions in the late 23rd / early 24th century. Though the transwarp drive clearly didn't pan out, they seem to be a ubiquitous design that could fit a variety of missions that the Connies usually handled (i.e. deep space exploration, diplomatic functions, and, of course, cataloging gaseous anomalies), and maybe a few others that the Connies couldn't. If you're gonna have ships either exploring uncharted space, or patrolling the borders of some of your more dangerous neighbors (Klingons, Romulans), it pays to send out something bigger that can take care of itself. The Excelsior was probably more than a match for a single Klingon battle cruiser. (As for Romulans, we can only speculate what a late 23rd century design wouldn't looked like, though I remember seeing one in a comic that I really liked).
Their frequent appearance in the 24th century indicates Starfleet has a LOT of ships of this class, and those sheer numbers would probably explain why the Ambassador and Galaxy classes didn't completely replace the Excelsior, the way the Excelsior replaced the Connie (By contrast, "Tomorrow is Yesterday" suggested there was only about a dozen of those).
The Soyuz, which was probably an offshoot of the Miranda, may have simply turned out to be an evolutionary dead end in starship design, or was built for some mission profile that was no longer necessary by the 2280's and didn't prove as adaptable as its Miranda cousins. (Though it is interesting that the
Bozeman apparently resumed service after emerging in the 24th Century, but you can chalk that up to Braga's desire to give his hometown a shout-out whenever possible
).
(Clearly the Klingons have the same attitude, given the lifespan of the BoP and battle cruiser designs, and these are ships that probably see combat far more often than 24th Century Starfleet did pre-Dominion War)
Oh, don't even get me started on Klingon designs that never change...right along with their costumes and weapons that never change either
And again, newer Starfleet vessels could have been re-used just as much as the Ferengi, Warbird, and alien-of-the-week designs. They chose not to build them because they chose to use the movie models instead, just like they reused the BoP and K'T'inga stock footage for Klingon ships, even though they also were quite out-of-date designs for the time period.
If the original CGI VFX are not up to HD standards, then CBS will have to update the effects for a BluRay release. So it's more a question of if they're going to put DS9 on BluRay at all. (Although I'm sure they will; the heavily CGI space scenes were only in the last two seasons.)
You still have to justify the time and expense of building a brand new model. In "Yesterday's Enterprise" and "The Wounded", for example, it was justified. In "Encounter at Farpoint" and "Cause and Effect", apparently, it wasn't.
As for the Klingons, you can hardly be surprised that a culture that still fights with knives and swords might stick with other hardware for long periods.
And Starfleet likely sent EVERY ship fit for combat to the front lines, given the scope of the Dominion threat.
So tons of out-of-date Excelsiors and Mirandas were fit for combat, but only a relatively few Galaxies, Nebulas, Akiras, Sabers, and Steamrunners were as well? Not to mention ZERO Ambassadors, New Orleans, Freedoms, Niagaras, Challengers, Cheyennes, and the 20-odd conjectural classes?
And who's to say that the newer ships we saw in those battles weren't proportional to their numbers? Whether or not you buy into Gene Roddenberry's suggestion that there were only six Galaxies at the start of TNG (which is pretty reasonable, given this huge new ship that supposedly took twenty or so to design), it's safe to say they likely weren't a common sight yet in Federation space in the early to mid 2360's. Now clearly several more had been built by the time of the War, but given that Excelsiors and Mirandas clearly number in the dozens, if not more, it'll probably be a few more years before Galaxies are more common.
I do wish we'd seen some Ambassadors, but for whatever reason, they didn't decide to make a CG version. (The model may not have been as detailed as some of the ones built for movies).
As for the conjectural ones, there may simply not be that many of them built, or they may not be designed for combat (most of those classes includes a ship at Wolf 359, which didn't exactly prove the worth of any of the ships involved). We certainly never saw even one Oberth fight the Jem'Hadar.
Probably just too expensive to build a new module of the same quality as the rest of the model every time it appears.
