Basically, they are trying to wean 1D series photographers off cheap lenses...
Seriously? Yes, Canon definitely wants to wean people off of those cheap lenses. Wildlife photographers using the 500mm f/4L IS with a 2x TC should ditch that cheap piece of crap and buy themselves an EF 1200mm f/5.6L lenses...except that Canon discontinued those. Why did Canon publish the MTF curves for the 500mm and 600mm f/4 MkII superteles (are they 'cheap') with the 2X III extender for 1000mm and 1200mm f/8 lenses, then eliminate the ability of the pro line to AF with that combination.
I'm not voicing my support for their decision. I'm describing it as mainly business-driven, with perhaps the possibility of an engineering angle that is just plausible enough to give them cover. Of course, they haven't really said enough to convince people there is a legitimate engineering reason (though I suspect I know what it is, as I said earlier) and so we're left to speculate and complain in a vacuum...as usual!
I suppose I could understand how Canon pride (and, more importantly, trying to repair or reinforce a reputation for reliable AF) would disallow having a somewhat underperforming AF system at f/8 (compared to f/5.6 and wider), while at the same time tweaking their newer TCs to reduce AF speed to ensure accuracy. Actually, that seems consistent for me: Even if you have to buy a new lens, at least you shouldn't be able to complain about AF misses (if people do have trouble in this area Canon will most certainly catch hell for it).
The upshot of all this, ironically, is to put me even and other shooters (especially wildlife / sports combo shooters) more firmly in the camp of the Sigma 120-300mm f/2.8, which is priced and performs just about perfectly to fill the niche opened by crippling the AF system. It doesn't do anything to help the 500mm + 2X TC user situation, though.