Lets face it, high mpx is in demand, as proven by crazy demand for D800... its demand is almost solely due to its high mpx.
Where exactly to people get numbers for "demand"? I've searched the web up and down, and I have never once seen any useful, concrete information about exactly what the "demand" is for any Camera, D800, 5D III, 1DX or anything else. I don't think there are such numbers. Without "demand" volumes derived from some kind of careful study that properly tracks D800 and 5D III sales from a number of major retailers
...not just one or two like Amazon or B&H (not items manufactured from the manufacturers, as they make and ship without any precise
regard to how many might actually sell), no one can really say for sure what the global market demand for these cameras is. Until the companies release official sales numbers, I think its simply hearsay to proclaim the "demand" for the D800 is larger than the "demand" for anything else.
In the Nikon world the D800 is obviously the big baby, (and apparently for a relatively small group of hyperpixelpeepers in the Canon camp who think ISO 100 DR is the end-all be-all of everything in the continuum of photography.) However in the Canon world, which is arguably larger, the 5D III is going to be the big seller. On this forum alone there are dozens and dozens of 5D III owners already, and I can only imagine that in the broader Canon customer base there are far, far more 5D III's than anyone could know about with any concrete certainty.
Personally, I think proclamations of "demand" for any camera at this point is extremely premature, especially with the Olympics right around the corner. I would not be surprised to see D4 and 5D III sales surge the closer we get, especially if Canon doesn't release the 1D X until the day before the Olympics start. The D800, while it certainly has a lot of glitter and gloss with its shiny 36mp sensor...is rather slow from a frame rate perspective, consumes an ungodly amount of memory space, and is in general designed for an entirely different market than your average Olympics photographer. Time will tell, but I don't really see either camera radically outselling the other in the long run.