Interesting post, I have stated a few times that the D300(s) users may find themselves positioned as the D700 users were - arguably left without a true upgrade and needing to re-think where they fall in Nikon's line-up or re-think their real needs anyway. Changing the D400 from a DX mirror based camera to a non-DX mirrorless one would likely qualify.
The strongest argument I have heard for a D400 is that the modern Nikon line has only one camera with a high FPS - the D4 which is out of the price range for most people. Could Nikon intend people to stretch that far up to get over 6 FPS (without using odd crop modes anyway)?
Of course we have been hearing rumors about a D400 for what seems like forever now and nothing official has ever materialized. Will this be the year or will Nikon decide that something else will sell better and shelf it again (if it even exists).