I enjoyed that thread too and learned a lot from the exchange between Neil and Perrone. One thing I'll observe, though, is that their exchange mostly revolved around how to get the absolute best image quality under difficult circumstances, which I appreciate is a priority for both of them (Neil with his birding and Perrone shooting indoor sports). Another aspect of DX is the increased portability of a DX kit because the lenses can be smaller.
As an example, compare the size and weight of the DX Sigma 10-20, Nikon 10-24, or Tokina 11-16, with FX 14-24 or 16-35 lenses for wide angle. Or compare the DX 55-300 with the FX 70-300.
Are people who use these lenses uninterested in a DX camera with more of a pro build? Nikon seems to think that a smaller sensor means a more down-scale market, if their Nikon 1 cameras are any evidence. But look at some M4/3 cameras like the new Olympus OM-D, and the high-end lenses Olympus and Panasonic are marketing for their M4/3 cameras. I'm not sure who understands the market better, but if there's anything to the M4/3 strategy, that would suggest that there is also a market for higher-end DX. Which would imply a D400. Also, I'd think Nikon would want to have this product just to compete with Canon's 7D.
I've heard the idea that Nikon's FX cameras are an upgrade path from their entry-level DX models, but that is at best partially true because Nikon sells those cameras with DX lenses that are only semi-functional on FX. So, if you have a bunch of DX lenses you're not much more compatible with a D800 than you are with a Canon 5D. It's not an upgrade path if the stuff you have now doesn't work with the stuff they want you to upgrade to; it's a whole new system.
And, Nikon has some pretty good DX lenses. I'll include the 35 f1.8 and the DX macros, all introduced after the advent of FX, in addition to older lenses like the 17-55 and 10.5 fisheye.
I understand that if you want the absolute best image quality, get the largest sensor you can, and mate it with a huge unwieldy lens. I'd put the 14-24 and 24-70 in that category, in addition to long telephotos. But for many purposes DX gets great image quality, and there are trade-offs in size, weight, and cost that make it attractive to some. Lots of people justified their desire for a D400 just by saying "I want one," and that's reason enough for Nikon to make one (if enough people actually want one).
The closed thread was very informative, but I also think there is more to the debate on the future of DX than what that thread discussed.