Too true. The ongoing argument is more about why SF would keep using a 100 year old designs though. Still, once CGI became more common, they could have rendered new ships for late DS9, but continued using Mirandas and Excelsiors.
I imagine a big part of that was fitting in with the older VFX techniques. It'd be weird for there to be fleet shots full of Mirandas and Excelsiors (and Galaxies and Nebulas) because they were done with physical models, and suddenly they've all disappeared when the actual battle starts, and it's just the Defiant and a bunch of Akiras and Sabers.
It's kind of funny that the cost incentives go the other way, now; with different CG pipelines and software, it can be just as much work reusing an old design as making a new one, so now, rather than seeing a bunch of hundred-year-old ships and very few contemporary designs because those were the models that were available, we're not seeing 23rd century designs you'd expect to be around in Discovery because it'd require just as much effort to make a Kelvin-family ship as a new one, and the current people in charge don't think it'd be as delightful as I do.
So why there are no Constitution class ships in service by TNG era?
ILM hated the TMP studio model. As for why they didn't start showing up after the transition to CG, I don't think I ever saw anyone say, specifically. Could be they were busy, so they focused on making CG models of the ships they'd already established as being around in DS9, Mirandas, Excelsiors, Nebulas, and Galaxies (though no CG Oberths or Ambassadors), and the ones they inherited from First Contact, and never got around to filling out the CG fleet. Or someone upstairs might've felt the same way about the Constitution as the Sovereign, that it was a movie-only ship.
Logically, we probably should've seen more Constitutions aside from a couple wrecks, and I imagine they were out there, along with the Ambassadors, the Oberths, and the BoBW ships, because the TNG-era shows established a big fleet with a lot of design longevity. Like I was saying before, it's possible the Kurtzman-era shows will end up going big the other way, and every show will have an unreasonably unique set of ships, even when it'd make sense for them to cross over, because there isn't the economic incentive to make use of existing resources.