In a sense yes; for the smaller sensor, they are covering the FOV of those 500mm and 600mm f/4 lenses with lenses offering 350mm to 400mm, and everything in between: the 400/2.8 and above all the 200-400 f/4 VR. The latter is significantly smaller, lighter and far cheaper than either the Nikon 500/4 or 600/4, and Canon's IS versions are heavier still.
With DX format, the market for 600/4 is like the market in 35mm format for the equivalent lens, a 900mm f/5.6. And in the rare cases that it is needed, it will be got the way that 35mm users get 900mm FOV: by using a 1.4x TC (on a 400mm lens for DX), or by using a sensor with enough pixels and then cropping.
Bird photographers need all the reach they can get. And no the market for the 600 F4 on DX bodies will not be like the market for the 900mm F5.6 on 35mm bodies. That latter lens would be huge and prohibitively expensive. The 600mm F4 is still lusted after because it is just about portable and just about affordable, and works well with a teleconverter. A bird photographers greed for reach knows few limits.
BTW This is one of the most interesting threads I've read for some time, due to the absense of the "But a FF sensor is bigger so it must be better" nonsense. The argument that current Canon FF users will migrate to digital MF is interesting, especially given how many seem to complain about Canon's wide angle lenses.
But there might well be a market for FF 35mm digital once pixel densities get high enough that a DX offers no telephoto advantage. Assuming that they can reduce noise levels. The D200 is rather noisy at ISO 1600 and above.