I do think the 1DXIII will be the last higher end DSLR. We might see a lower end body, but I even view that as unlikely. All accounts I read is that Canon is trying to transition 100% over to the "R" ecosystem. No new EF lenses, no new EF-s lenses, and likely no new DSLRs. I also strongly suspect that the M system will be phased out as well. It is all eggs in the R basket, at least for interchangeable lens cameras.

That said, I fully expect, and all the rumors indicate there will be a "R1" that will be the successor to the 1DXIII. Perhaps as soon as 2021. When you consider the 1DXIII's performance in liveview and the R5/6, the technology really does seem to be ready. It is likely more just manufacturing supply chain holding it back.