There is one hard-stop here: if Canon wants the R series to be taken seriously, they need a 1DX2 equivalent before the 2020 Tokyo Olympic Games. That is a showcase not just for the athletes but also for the photographers; Nikon, Canon and Sony all out-do themselves for the pro community. And it's on their home-turf. It would be embarrassing if Nikon had the Z7-mark2 and Sony had the A9iv out there while Canon didn't have an ultimate R ready to go.
I'd expect the CPS ready room to have rows and rows of the "1Rx" (have to call it something) with plenty of fast, action-driven RF lenses -- a 70-200 f/2.8, a 400 f2/.8, etc. You don't want to adapt EF lenses to your 1Rx at the Olympics, do you?
So Canon can have their 5D4-minus R, and their 6D2 equivalent R, etc., and build out the line in 2019. But for the Olympics they'd better be ready with the camera the pros want to use.