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.