The only logical reason to use mixed SD instead of dual CF on the 5D was to begin the inevitable transition of their high-end DSLR line away from the CF standard to the more commonly available SD standard
Forgive me, but how is that the only logical reason? That's just one potential reason, about which I question the logic. Were it a UHS compatible slot I could maybe see it, but the notion people are going to go out and buy a bunch of high speed SD cards to stick in a camera that can't write at high speeds is illogical on its face. Slow SD doesn't precipitate a transition to fast SD.
In any case, I hope you're wrong; I don't want to move to SD. I hope they go to XQD.
Personally, I always assumed the second slot being SD was a packaging thing, i.e. fitting two CF within the footprint of the 5D grip wasn't viable.
so they crippled the SD slot because they presumably couldn't cripple the CF card slot for some reason.
Oh come on. You honestly think Canon said:
1) I want people to buy into SD
2) So I'll add an SD slot to the 5D
3) But the 1Dx has two slots, and that's a selling point, so I'll make one of the 5D slots slow
4) Since I want people to buy into SD, I'll make the other slot slow, but wait, I can't figure out how! Never mind, I'll deliberately hinder the the slot I want people to get used to using.