A few things: As big as they are, I think it's important to remember how small they are in comparison to many other sci-fi capital ships. If they're difficult to build, I think it's because of the level of technological sophistication, not raw materials/factory space. We're talking about the combined manufacturing abilities of 150 worlds -- maybe something like 100,000(!) planets/moons/colonies/outposts/habitats/stations/etc (never mind ships) and a trillion or three people. We don't really know what Starfleet's immediate needs were or how they were addressing them. The Intrepid was not a specialized combat vessel -- it just reminded us some of what peacenik Starfleet ships can do. (Its most unusual components were its fast 9.975 cruising speed and bio-neural gel-packs for computing capabilities. Then we could mention its EMH and new nacelles to combat the effects of warp on space.) But it wasn't taking over for other larger ships. It wasn't the new Enterprise heavy cruiser or Nebula frigate. It was a light cruiser, taking over for whatever previous generation of vessel fit its profile. We know that they needed one specific thing -- a way to combat the Borg. But it doesn't seem as though Defiants became all that common, nor more successful than other platforms that adopted its testbed technologies -- i.e. the Sovereign, maybe the Lakota, etc.
The Galaxy-class is large for the crew it has. You could quite easily walk an entire deck without ever running into any other being there is so much space inside. Now if you bump up to the Universe-class starships (USS Enterprise-J) in the 26th century, we are going towards other sci-fi capital ships in scale since its two miles long and a good part of that wide as well.
What's more interesting is how all these classes of vessels (nebula, new orleans, etc.) were made with a base look and then that morphed into the Galaxy class.
Is that what happened? Since we saw the Galaxy first, I always assumed that was the first and the rest came after.
Actually, the Intrepid-class is a similar size to the New Orleans-class which is the "Galaxy-family frigate", the Nebula-class is about a third longer with a much wider beam so if anything the Intrepid replaces the New Orleans-class (and potentially older Excelsiors like Excelsior, Tecumseh, Fearless and Repulse). IMO, the Soveriegn-class is the new (medium to heavy) cruiser designed to somewhat replace the Nebula, on-screen canon hasn't given us a Galaxy replacement yet but TrekLit's Vesta comes close (and Treklit's Luna is another Nebula replacement) and STO has several.
We don't have any true timeline cues, other than the registry numbers. So we might just as well go with those. So... Our Galaxy benchmark is supposed to be the class ship NCC-70637, while the lowest Nebula registry is NCC-60205. The lowest numbers out of the Galaxy-kitbashed lot at Wolf 359 go to the Springfield class, with NCC-57302 in evidence, well below the lowest Galaxy or Nebula numbers. The Springfield "still" has those bulky, angular marker pen nacelles, though. The Challenger has Galaxy nacelles combined with a custom saucer, at NCC-57580. So does the Niagara, with NCC-59804 for the Princeton shown. (The supposed sister ship Wellington with a much lower registry I sort of visualize as having had Ambassador nacelles "originally", considering Ambassador is where the custom saucer elements and secondary hull come from.) The lowest Akira, with Galaxy-like hull curves, is NCC-63549. (Indeed, that's probably the only number ever painted on any Akira CGI.) The New Orleans is a totally Galaxy-based 'bash, at half the scale, and with NCC-65491 shown. The Freedom again combines Galaxy nacelles to a custom saucer at NCC-68723. The Ahwahnee has a custom saucer and marker pen nacelles at NCC-71620, but probably is as much a Galaxy family member as the rest of the lot. Odd that the "older" nacelles persist there... So I'd say there was a whole family of these things, starting out small just in case, and also starting out gradually so that the earliest designs still had the previous generation of warp engine, or were created by putting the new warp engine on the previous generation of hull. The Galaxy only came when the design ethos had found sufficient maturity. FWIW, only the Nebula registries here are all over the place. The other Wolf 359 kitbashes fail to reappear so there's no spread there; the Akira, while looking the part, and ubiquitous in later Trek, never flaunts the alternate registry numbers; and the Galaxy stays nicely compact around NCC-70000/71000. This doesn't mean we should think that only the Nebula enjoyed a long production run, or that all the others were built in very small numbers. OTOH, nothing stops us from thinking exactly that. Timo Saloniemi
The Akira class and other First Contact ships all had registry numbers that would place them as being launched in the 2350s, which I think is way too early, considering they all follow the more "modern" Starfleet aesthetics of the 2370s.
For the Excellsior, as said, no cannon for saying if it worked or not, and then Tng and Voyager turned Transwarp in to borg conduits, or Voth/salamander drive to pass warp 10, even read it was based on an idea aout when the Enterprise went through innerphase space in Tholian web, basically phase into innerphase like a hyperdrive. Whole bunch of what if's. Whether it worked or not, the excellsior class was pushed in to service, and aparently done quite well. Galaxy is the biggest (volume wise) ship we've seen in starfleet so far, its Huge! I would say its like the Navy, you have only a few big ships, like a carrier, buy plenty of destroyers, subs etc. You could also say you could build 2 or 3 Intrepid classes for the resources of 1 galaxy. So you may not see 100's of galaxy's running around like say an intrpid, but maybe dozens in its initial run. Now with the advent of slipstream, the quite wide galaxy doens't lend itself well to it.
How so? The Akira is the only one that shares aesthetics with any recognizable era - the saucer is quite Galaxy-like, the color matches as well, and the engines have the same sort of forward domes and general shape. The Saber and the Steamrunner are dark and angular, unlike anything else we've seen. So, quite possibly the fashion of the 2330s-2340s! (The Norway is sort of neither here nor there - she would be right at home in TOS, really, with the plain lines and the grey finish.) Timo Saloniemi
In my head cannon, my UFP / StarFleet Doctrine has a standardized Fleet Size that is doubling of # of Ships produced for each class as the overall size goes down in the primary fleet. Ergo the largest ship will only have 1 copy per Fleet 2nd largest ship will only have 2 copies per Fleet 3rd largest ship will only have 4 copies per Fleet etc. For my Fleet Doctrine, it works for what I want it to do, especially given that I have "Millions of Fleets" Each Fleet has 1023 vessels per fleet with LOTs of Fleet spread about the Milky Way Galaxy in all 4x Quadrants given how far the UFP has expanded by the start of the 26th Century and how many areas we need to protect / explore along with the vastness of space.