Right. FX telephotos are great for DX users. And my Fuji 100-400 and 150-600 weren't really any smaller than their Nikon FX equivalents.
The problem with Nikon DX is outside the telephoto range. They need a 12-20/2.8, 10-24/4, a 16-50/2.8, a 16-80/4, and some wide compact DX primes, like a 16/2.8, 18/2, and 35/1.4, to make DX an asset instead of a liability.
Fuji's APS-C system is great not because it has lenses that simply exist, but because they take advantage of the sensor format. For example, if you want a 28mm-ish prime on Fuji or Nikon DX, a Fuji user can choose from an 18/2 or 18/1.4, and a Nikon user can choose the 20/1.8:
View attachment 109956
Compare that to the 35mm-ish situation, where there's much more parity between Fuji's 23/2 and 23/1.4, and Nikon's 24/1.7:
View attachment 109957
Finally, let's say you want a fast 50 for Fuji, Nikon DX, or Nikon FX. For DX you need to use the FX 35/1.4, which is very slightly
larger than the 50/1.4:
View attachment 109958
Nikon has, and must do better for DX users if they want to promote it as an alternative system to Nikon FX, and compete with Fuji.
In my opinion, Nikon sees DX as a gateway to sell users on FX. I think that's wiser than trying to promote two full systems, but I'd go one step further and suggest that Nikon would do even better to attempt to cut costs on the Z5 even further, and abandon DX entirely. It's not worth the R&D time to make a DX lens lineup when they could be improving FX, where they have a potential competitive advantage over every other system.