I doubt cfast will be used anymore by Canon, I hope.Should be interesting to see the relative directions Nikon and Canon go with their new pro bodies. I can't see myself investing in a 1DX Mark III at this point but who knows. There would have to be some pretty compelling advantages. Those insanely expensive and increasingly obsolete CFAST cards are going to be yet another waste of storage dollars I imagine.
I would expect the D6 to be similar to the Z6/Z7 bodies where you have a hybrid CFExpress/XQD slot.Physically they are same(Type-B) but use different protocols. Xqd is pci-e 2.0 interface with some unknown protocol while CFxpress moves to PCI-e 3.0 interface with NVMe protocol (electrical interface being same betn two many devices are capable of supporting CFxpress via firmware upgrade.). Both CF(IDE) & CFast(Sata) are dead moving forward with introfuction of CFxpress. Since Canon absolutley loves antiquated standards(Micro B connector on 90D with USB 2.0 controller) we can rest assured that replacement for 1Dx Mk 2 will come with atleast 1x CFast or CF slot unlike D6 which is moving away from 2xXQD.