Heck, even 5DIII's sales would suffer because of it's increased price (highly advanced AF would obviously cost more). Why would Canon take such a risk?
Its increased price? What, like the D700 was significantly more expensive than the 5D MkII? Higher production cost and lower profit margins are more likely, as well as the fact that they'd risk 1D X sales.
I'm not going to revisit old ground (I think that Neuro and I will have to agree to disagree for now), but I would predict that the 5D MkIII will have the 7D's 19pt AF system. My reasoning is that it's inferior enough to the 1D X AF system, already developed (hence no R&D expenditure), and is used on the 7D; thus it would become Canon's new mid-market standard.