Well the D500 is the only one which has large mass market appeal.
Canon only uses CFast 2.0 in the 1DX II and some cinema models. Arri also use CFast 2.0.
I would guess because of the D500 that XQD should easily outsell CFast 2.0, unless filmmakers simply consume a whole lot more cards than still photographers?! Or do most D500 shooters just use the SD slot?
Since the future CFExpress appears to be basically a next gen XQD, it seems likely that in the future more camera makers will get on board, if resolution and speed keep increasing. Present day XQD is already a delight but no doubt with 100MP files it may start to feel a little sluggish.
SxS seems to be of the same capacity and speed as current XQD. Sony do not seem to have any problem making different card formats for only 2-3 camera models at a time.