it is to my understanding that Starfleet claims to be a "humanitarian" fleet.
I also don't think -D server as a military important ship, but is rather an Diplomatic Envoy, as its essentially a glorified space liner with phasers and photon torps, though I would say this a poorly though out title for -D.
Thirdly, before Borg showed up, Romulans revealed the new Warbirds and Klingons but the Vorcha, it was one of the most advanced ships in Alpha/Beta quadrant.
it is only after these encounters that federation starts actually looking towards war ships like Saber, Akira, Steamrunner and stand outs like Defiant and Prometheus.
Starfleet in general seems a bit incompetent or underrepresented in TNG, with there being Fed colonies that haven't seen ships in years, and as I understand, while Federation might be massive from one side to the other, a lot of stuff in the middle are unincorporated planets that either didn't want to or were not eligible to join.
In the first season or 2, Starfleet lost a sister ship of Enterprise to some NEET living in a space hole and got their HQ taken over by Proto-go'ulds>>108355395>That shit hardly ever happens in a navy except during a major crisis
That is every other episode though.