I don't see how R&D is an issue with a potential D400. I'm not saying there isn't any but the body that most of us (who are interested in posts like this one) desire is hardly a re-invention of the wheel. Most of the desired components have already been developed and are currently in use in other bodies.
You logic regarding the market might be correct. And if Canon arrives at the same conclusion then indeed it might be a wise course....from a financial standpoint. It will still not be a good decision from a brand loyalty standpoint. There seem to be a lot of disappointed and disillusioned D200/D300 owners out there. Nikon is counting on them to move to a Nikon FX body or settle for a D7100. For those of us, however few we might be, who don't want to move to FX and who have never had to "settle" for any Nikon product in the past, this will be a new and not so happy Nikon experience for us.
BUT, if Canon decides to tap this market then Nikon will lose that market share. And while it may be small, companies forced to make hard product line decisions usually can't afford to lose any.
In the end, for me at least, it really just comes down to what I mentioned above. In all my time as a Nikon user there has always been a body with just the right features for just the right price. I've _never_ felt like I had to buy more than I needed or settle lower.
But that has not been the case these last couple of years and posts like these are rife with advice to buy more than what I need or less than what I want. Kinda sad.