I have no idea what the D400 would be like, but I do not agree with you for the following reasons. The D7K has a Magnesium alloy body with all the sealing of a D300, for example. It is even rated with the same number of shutter releases as the D700 (175,000.) The Leica M9, for example, is a much smaller and simpler camera than most Nikons and yet it is considered a “Pro” camera. In fact, Leica just put an APS-C sensor in the X1, which is slighter bigger than the D-lux 4/5.
The advances in technology are for faster, lighter and faster products. Remember the first cell, the Dyna:
From my perspective, I see no reason why I should carry a DX camera of the same size and weight of an FX one. What is the advantage of that? If I opt for a smaller sensor, a compromise, then I should get the advantage of having a smaller camera.