We've been told a couple of times in very vague terms that Canon may be looking to replace the EOS 5DS and EOS 5DS R with a higher megapixel EOS R series camera.
The high megapixel 5DS series from Canon is a niche product in the lineup and it would make some sense to move this series over to the EOS R side of things. I don't believe the average 5DS user would be all that bothered by adapting EF lenses, especially with the adapter options available to EOS R shooters. Landscape shooters will likely love the circular polarizer and ND filter adapters available from Canon.
We believe that a native wide angle zoom will be coming sometime in 2019, and that would help with the transition to the EOS R system for shooters that don't want to go the adapter router.
Take this with a grain of salt, as I'm not sure Canon will be ready to kill off a DSLR line this soon in favour of an infant full frame mirrorless system. That said, I have no information about how well the EOS 5DS series of cameras sold, so this could be the obvious line to kill off first.