Well, one would think that something as insanely complicated, large and groundbreaking as NX-2000 would have to wait a long time for series-produced sisters. It sounds perfectly reasonable that there'd only be a handful of ships in the 2XXX slot, to be followed by the first series production batch a decade or so later, with those 14XXX registries. Quite possibly something similar happened with the Constitution class, too, as we get the NCC-1017 in addition to the ships clustered around 16XX-17XX.
Whether there would still be demand for a second batch at 42XXX is debatable, but I don't really see why not. Those ships would still have lower registries than any witnessed "new" ship type save for the Ambassador class, and I don't think the giant Ambassador would be a direct competitor for the Excelsior niche, but more like a high-end companion. The next "new" designs emerge at NCC-52XXX, which is where the earliest Steamrunner registries lie, and then comes the Galaxy generation with its earliest examples at NCC-57XXX (the New Orleans class).
The one Excelsior with a registry even higher than 42XXX-43XXX would be the Melbourne, which originally in "Best of Both Worlds" was a Nebula class ship and thus a plausible recipient for a high registry. Too bad the registry wasn't changed when the ship became an Excelsior in "Emissary". Then again, good that this new ship's registry was never quite seen on screen and can be ignored...
Apart from the Excelsiors, the Mirandas also appear to come in two batches - 18XX and 31XXX. And the Ambassadors and Niagaras also have two batches, it seems - but that's just fine with me, because we actually see that there are two distinct variants of Ambassador, and we can also speculate that there used to be a variant of Niagara that had different engines, considering that the lower Niagara registries would (in the chronological interpretation) coincide with an era when those Galaxy engine lookalikes probably weren't available yet.
All in all, I have no problem with the Okuda registries in the TNG era. It's just too bad that they trample on "established" fandom registries in the TOS era - but happily, Okuda hasn't done too many TOS era ships. And the ones that were given Okuda registries in TOS-R didn't all get to show the numbers on screen and in focus, so there's wiggle room...
Timo Saloniemi