Well, for comparison's sake, a modern ship the same payload capacity of the Miranda
is actually the Sabre
, which is a fair bit shorter with a crew complement in the 50ish range. So it doesn't seem unreasonable to me for a Miranda
's modern complement to be pretty small, with a greater proportion of civilian specialists and the like than in the 23rd century.
I think the reuse of older designs is mostly an artifact of industrial processes predating industrial replication. Essentially it was easier to make hull frames and outer hulls in a certain shape, and the parts to do that with Miranda
parts was comparatively easier than starting over from scratch. This would make more sense, too, if the majority of ships or hulls were built in civilian yards who might not have the latitude in operating costs to switch between "patterns" willy nilly. To some extent I even think some of the wilder ship designs we've seen post-CGI era are basically made that way because the shipyards designing them were recycling hull component shapes that were easy for their replicators or fabricators to make.
So when Starfleet gave away a ton of its own old shipyards/drydocks/fabricators to civilian contractors in favor of building brand new ones, you'd then make some inertia for the smaller contractors to just keep using them and find ways of updating the internals.
In fact, I'm somewhat inclined to argue that's how something like Lakota
came about. A proof of concept of modernizing Excelsior
hulls and allow reusing of facilities/tools that were used on it. (And Leyton using a smaller, out of the way dockyard owned by a small company would make it easier for him to get away with doing it in secret)
Also, Rick Sternbach in the Klingon Bird of Prey Owner's Manual
makes a similar claim that the Klingons only reuse external frames or designs instead of toying with new exteriors because there's not a huge benefit for them. Instead they just build the basic hull, give it their newest engines and then hand them/sell them to the Great Houses to outfit with technology the way they see fit.
I think a similar thing happens with Starfleet ships but in reverse. Contractors build the hulls, then hand them off to Starfleet to fit out with mission gear in big, enormous starbases like Spacedock.