Per Mike Burnhil, Canon UK CPS said they didnt move to CFast because they dont want to annoy customers. That's why the chasis of the 5DS/R are identical to the 5D Mark III.
Fast forward to 8:20 http://www.fotosidan.se/cldoc/video-interview-canon-eos-5ds-and.htm
Another interesting point is starting with the Canon EF 70-200mm f/2.8L IS II USM lenses announced from 2010-onwards are designed for high pixel density sensors. Tough luck to me then.
Like you I would love to transition to CFast but who wants to pay $180 for a Lexar 32GB memory card? Yes, it is 3400x (510MB/s read) but still.
I would rather see Canon make the transition to CFast with their Cinema EOS line andlater transition to CFast once $/GB be at par with CF cards of today.
Mike also mentions high pixel density camera makes lousy video cameras.
Again, looking forward to the 5D Mark IV hopefully with 4K, modest MP bump, Sony-level DR and all the bells and whistles in August.
One of the biggest disappointments I have with this announcement is Canon was LAZY and didn't move up to using CFast. You'd think with files that big they'd want to go CFast, which has been a standard for a long time now. Or at least dual CF UDMA7, I mean, come on!
And, what'd be a really neat trick, is now that there is USB3 (yay!), if they were to implement raw video using a computer with SSD(s) to stream through the USB3. Well...crunching the numbers, that wouldn't work. Assuming 30 fps, 64MB per frame, that's 15.36 Gigabit/sec needed. Thunderbolt 2 could likely manage that, but we don't have it on the camera. Perhaps a crop-mode, or an s- or m-RAW format would work though.