The EOS R Pro model from everything we have been hearing will be the successor to the 5DS/R. Not the 5D4. The 5D3/4 line are the wedding professional's workhorse and they don't want a super high res model as it is more trouble/data than they want/need. The 5DS/R bodies were specialty cameras the EOS R Pro would be most apt to replace. There will still be a 5D5 DSLR body as there is WAY too much glass out there already that won't be replaced anytime soon. The EOS R was meant to be a sidewise to help transition to MILC by using the same 5D4 sensor.I agree with the logic, but I am more so wondering if that's a marketing priority. It seems like Canon likes to give each and every model its time in the lime light before they introduce something new, and the 1DX III is likely the priority for the olympics.
On the other hand, part of me wonders if the timing of projected release is telling here. The 1DX II preceded the 5D IV by a few months in 2016, meaning that in following the same schedule we should then expect a 5D V some time in 2020 following the 1DX III. However, now we're hearing that the high resolution body will come next, potentially in the time frame we would otherwise expect a 5D V. This makes me wonder if the high resolution R is the true spiritual successor to the 5D IV, and the EOS R is intended to be a down-market version of the 5D for the "do everything pretty well" niche. If the 5D V release time slot is potentially occupied by this resolution-focused R body, when or will we see a 5D V? Interesting times.
MILC bodies don't offer any massive "upgrade" to DSLR for full time pros. They just don't. This will be a slow transition by Canon that will last the better part of a decade.