You may be right, I'm sure Nikon know what they're doing. Subjectively though, the D300 did seem to sell well and the D300S is still listed as a current product. Of course, selling well and making money aren't always the same thing.
The D300S may be listed as current, but I don't think that there is much stock around. I know my local Jessops haven't had one on display for a very long time, but the D90 was there until quite recently. What Jessops put on display is no big indicator of what sells, and what is available, is does show the marketing strategy, as all the Canon DSLRs up to the 7D are on display. It seems Canon may have stock of those camera available to sell. I also think if D300S were plentiful the price would have come down more. As it is the price is staying pretty constant and may even be starting to creep up. Maybe as supply goes down. :shrug:
As for what a D400 should have, imho of course.
A very similar body and build to the D300S/D800.
Dual card slots. Seeing as the XQD format seems a dead duck, CF and SD slots.
18-24MP.
8-10 frames per second. (maybe 8fps as standard, and 10 with a grip and more powerful batteries.) Whether those last two can be done with the current technology is another thing.
The AF from the D4.
An LCD to match the D800/D4. With any colourcast faults fixed of course.

A video mode to match or improve on the Canon 5DIII/1DX. (I don't use video, but if they don't match or improve on Canon offerings they are dead in the water for Pro's or any serious enthusiast)
ISO to 12k - 25k extendable to 100k
Seeing as it would be the 'Pro' DX camera, it should have a lot of the features of a D4. I don't think it should have a D4 style body, with integrated grip as that would alienate the majority of buyers imho. Those that want that style can buy a grip. Not exactly the same, but close enough for tha minority who want that style.
They've kept people waiting for so long that there may not be a D400/D9000 ever appear. If that were to be the case, I would probably be tempted by what Canon have lined up to replace the 7D. Seeing as that has just had a major firmware update, the shelflife of the current model may have been extended, and a replacement (should there be one

) may be quite far off.