Thats an interesting take. I suspect the market (Nikon) will determine the need for f/8 AF, and that if Canon wanted to adjust their firmware to enable f/8 AF while preserving full capability /performance at f/5.6 they could do it. High performance AF may be the differentiator that Canon is going for, though, and for the pro series bodies they could win the argument on the technical merits, i.e. if what you say is true then not many pros will jump brands just to get poorer AF performance at f/8