It might be that some of those older Miranda etc... ships, were older ships that werer in mothballs (fleet reserves etc..) and reactivated to boolster the fleet.
Someone else on Youtube suggested the same thing, but I'm skeptical of that, simply because of the sheer number of Mirandas we saw.
I tend to see the Miranda, like the Excelsior, as a multipurpose design that was used for such varied uses as surveying already-charted planets for things like colonization or mining (Reliant
), science vessels (Brattain
) or a supply ship (Lantree
). Most of them would've been pulled away from these duties and sent to join fleets at the start of the war.
Maybe they just never built very many to start with. Therefor you don't see them for the same reason you don't see the newly created Sovereign Class, at the time of the Dominion War there are only two or three of them.
The absence of Sovereigns never bothered me, in the real-world sense that they might not want to show hero ships from other shows/movies. (We also never saw another Intrepid other than the Bellerophon
I'm curious what exactly the cost (both in money and time) of creating a CG version of existing ship designs was.