I think you're living in a fantasy land if you think there will be a 1DX replacement anytime in the next three years, if ever. I'm on record that it's the last top end, professional, conventional DSLR Canon will ever produce. To the extent it is upgraded, that will happen through code -- firmware.
The 1DX is so advanced at this point there is little they could realistically improve upon. What can they possibly offer that will induce high-end stills shooters to dump their $7K camera for a new $9K camera? Combine that with quickly changing technologies, and the industry is simply going in another direction altogether. Five years from now the 1DX will still be the top still camera with video capability.
The future will be video cameras with stills capability. A whole different world is emerging.
Speaking strictly as an enthusiast who does not earn a dime from my camera, the 1Dx will be the last FF camera I purchase (barring theft or some other catastrophic loss).
For stills (subjects in motion, landscape, low light), I am so pleased with the range of pictures I can take. I can't see myself ever needing a reason to upgrade to another DSLR, even if said camera is a mkII!