The other problem with that argument is that they aren't proving f/8 AF sensor, making the TCs occasionally useless. Added pixel density doesn't have that problem.
See, you've struck upon something that has nagged at me about that lack of AF at f/8.....
Since the 1Dx is now a FF camera, 1D shooters have "lost" the 1.3x tele effect of the crop on the APS-H sensor. As a result, if you've been shooting at major sporting events with a 300mm or 400mm lens your net effect is:
390mm on 1DIV to 300mm on 1Dx, 90mm less effective reach
520mm on 1DIV to 400mm on 1Dx, 120mm less effective reach
(note, I've neglected accounting for the effect of going from a 16M to 18M pixel sensor on the overall cropping ability due to more pixels, but it's not a major factor in the results anyway)
Suddenly, as a pro sports or wildlife shooter, you find yourself with 30% less reach than you're used to shooting. That's not chump change, especially when you consider to make up for that a pro shooter using a 300mm in the past would have to go a 400mm lens, along with the added weight and significant costs.
Whats worse....if you've been shooting f/5.6 long tele-lenses, turning to a 1.4x extender to make up the difference puts you at f/8 and you lose AF when that happens. You don't have the option of going to another lens to make up the difference in some of these cases, either due to cost, weight, size, or the fact that a longer lens does not exist.
Part of me wonders if this isn't why Canon improved the 1.4x and 2x tele rev 3's as much as they did and specifically so for the super-tele lenses. They accounted for the move to full-frame and to ease the pain so to speak in the lost of the 1.3x tele crop effect, seriously improved the extenders for this reason, and gave shooters a reason to purchase the pricy 300mm and 400mm f/2.8 rev II L lenses.
Think about it...... if I was shooting a 400mm f/5.6 on a 1DIV (520mm f/5.6 effective), on a 1Dx that's a 400mm f/5.6. To make up the loss of reach, if I add a 1.4x tele, I get a 560mm f/8. I make up the loss in reach with some to spare, but I lose AF.
My options? By the "new" 400mm f/2.8 IS L II, add the 1.4x tele rev3 "specially enhanced for that series of lens", and BANG!!! I have a 560mm f/5.6 lens.....at the budget busting cost of $8000 (not to mention the $6800 the 1Dx cost me as well).
End result...Canon sells a new body, new lens, and new tele-extender so you, the pro-shooter, can maintain the reach and AF capability you had previously. All three for the low price of $14,800.
I'd bet that has a lot to do with the rumors and statements that Canon is re-thinking the loss of AF at f/8....a lot of big shooters and agencies are probably not very happy about this.