You might have only needed a 400mm lens on a crop but now you need a 600mm on a FF so it encourages sales of more expensive lenses.
That is simply not true. A 400mm lens on a cropped FF image is exactly the same as 400mm on a crop sensor. The only [current] benefits of a crop sensor is sensor/camera production costs, cropped image resolution, image, fps, and lens size/weight.
It will always costs less to produce a smaller sensor, but overall costs are dropping and as more people make the jump to FF, the economies of scale will drive a more rapid decline in costs. Cropped image resolution is a valid issue, but the 1Dx and D4 prove, for most people, that image is not about mp, and this truth is underscored by the resolution of cropped D800 images. Fps is only a matter of processor and storage throughput and capacity, and that is rapidly changing too. While the current D800 may be "slow" in that regard, I would be very surprised if the next iteration did not provide for 6+ fps, and I suspect the same will be true that Canon's first or second "big" mp cameras will also solve or come close to solving the fps issue. As for weight, that only applies to lens and you will still be able to use your older crop camera lenses if you really want to pretend you have a 600mm lens by using a 400mm on a crop sensor.
I don't know what Canon or Nikon will do in the future regarding "pro" cropped sensor camera, but I would be very surprised if they aren't looking looking at the FF "problems" the same way as am I and recognize the inherent long term economies of abandoning pro-level crop lenses and bodies.