On the other hand, the Excelsior 2000s may well have been ordered prior to the Federation 2100s, but since the 2100s were using well established technology, they went together quite quickly, while the experimental nature of Excelsior demanded much much more time. It could have been on the boards for 15 years or more before finally being realized as a functional vessel.
I just now thought of an idea that I kinda like: Suppose they authorized the 2000s and the 2100s at the same time as two classes of ships which were meant to have similar capabilities. The 2000 series would be there to be the next generation bigger/faster/stronger class and operate much more efficiently than the current generation, but they knew that it would take a long time so they authorized the 2100 series to do roughly the same thing, using well-known extant technology, which won't be nearly as efficient as what's on the drawing board, but can be built next year. This would make the Dreadnaughts a stop-gap between the concept of the Excelsior and the realization of it. It would also explain why there were evidently so few dreadnaughts (just one mentioned on-screen) but so many Excelsiors over the years.
Let's not forget that Sternbach's TNG Technical Manual tells us that the E-D took something like 20 years to put together.
On the other hand, FJ already gives us an Excelsior, a Bonhomme Richard-class Heavy Cruiser NCC-1718, though there is no NCC-2000 anywhere listed... However, if you go with the Mastercom idea that most of the FJ ships were authorized but never actually built, then maybe NCC-1718 was one of those that ended up on the chopping block, but the name ended up being applied to previously unnamed experimental platform NX-2000.
--Alex
This is one reason I like to treat registries as "batch" numbers that aren't necessarily meant to be sequential, but to be based in some measure on the specific ship design or similar factor. Interestingly, I think it also fits well with FASA's TNG-era take on the fleet captain, who would have been a flag officer between admiral and commodore whose job would be to oversee specific ship developments or special projects, such as the Excelsior and the Galaxy.