Final nail in the D400's coffin?
How can something that has never lived die?
Stillborn. According to Thom Hogan the D400 has been floated around numerous times at Nikon over the past few years, and even now it's being considered. He also says there's internal politics at Nikon that replicate the debates on Internet forums regarding whether Nikon should just do the D7000 series with no D400, or not -- apparently different design groups work on each camera and have what amounts to turf wars, the thinking of the opponents of the D400 being that D400 sales would take away from D7000 series sales (which is half-assed backwards given that a D400 would net more profit per unit than a D7000-D7200, and that's also according to Thom).
The real problem with the concept of using a DX sensor into a D810 body is that you'd essentially have the same image quality and autofocus performance as the current D7200?
Image quality, maybe; AF performance, definitely better. The entire point of the concept is that a D400 represents an improvement over the D7200 -- not only in terms of build quality, but also in terms of fps and AF. BTW, it's easy to have better AF than the D7200, it isn't even Nikon's current best AF. As for image quality, that is also a distinct possibility, and if nothing else a D400 will have better image quality than the 7DII.
Buffer is no longer an issue as it was with the D7100,
It actually is an issue, as is fps and the fact that getting to 7 fps requires a 1.3x crop mode. Since that's 1.3x crop of DX mode, that also effects image quality, so a D400 would have better image quality based on that one thing alone.
so the only real selling point would be higher frame rates. The bigger, more cumbersome body is difficult to justify to new users
It's not a camera for "new users." This is the fallacy that many D400 naysayers keep putting forward. A D400 would be a specialized tool -- to say it doesn't appeal to "new users" is like saying a Nikkor 300/2.8 doesn't appeal to new users, it goes without saying.
or people trading up from the D7x00 series.
Properly implemented, it would have appeal for some D7000 series users, especially those that settled for a D7000 series camera because there has been no follow-up to the D300/D300s (we see them often in this forum).
You really only have a tiny D300S replacement market and the few buyers who want a DX backup for their D810.
It's not a "tiny" market. That is the biggest fallacy put forward by the D400 naysayer crowd. I believe even Nikon knows it's not tiny.
I look at the current 7D II and I see a really bloated crop body,
Obviously you are not the intended buyer, but don't make the wrong-headed assumption that you speak for everyone, or even necessarily for a majority -- for instance, the majority of people using cameras these days are using the one in their phone, and they would tell you that any camera other than that one was "too heavy" and "inconvenient" to use.
although if I was a dedicated Canon user, it might be attractive at current pricing since the 70D feels like an upsized plastic Rebel. On the other hand, when the 7D II is only $50 less than the 6D, the value equation for the crop body really suffers.
See, you are confusing the sensor with the body. Sure, a good FX sensor has some advantages over a good DX sensor, although I think most overestimate it and don't realize the trade-offs, but likewise a well featured pro body has a lot of advantages over a less well featured consumer body. You are comparing apples and oranges here, and again assuming that most other photographers don't have the good sense to know any better about the value of a well featured pro body.
I wouldn't be a bit surprised if Nikon does produce a $2,300 DX body, since the outlandish pricing might be a reflection of the inherently low sales volumes of such a niche product.
You are absolutely wrong about that. Nikon could destroy sales by overpricing, just as they could overstimulate them with underpricing.
My guess is that after an initial rush of D300S devotees, a "Pro" DX body at that price point would sell in lower volumes than even the D5 flagship.
Your guess is based on numerous wrong assumptions, which is why it is so outlandishly wrong.
). And of course, they would have to give it D5 performance, or nearly so - it would have to outshoot any of the other FX options by a fair margin (FPS, buffer, AF performance).