How do you explain the fact that the D850 is clearly superior to the 5DMKIV/5DR, like the D810 was clearly superior to the 5DMKIII?
Bad examples - D850 was released two years after the 5Ds and a year after the 5D4, while the D810 was released two years after the 5D3. D800 vs 5D3 is a better comparison.
Regardless, the point stands. After the 5D2 and 7D were released, Canon's pretty much been focusing exclusively on video technologies and dual-use technologies. Even the 1Dx2's AF system is materially similar to the 7D's - there's just a lot more of it. They had the lion's share of the market, and were happy to coast along while developing the technologies required to dominate the next era. At that stage - even before Sony had made a full-frame camera - they had identified Sony, not Nikon, as their real rival, and worked towards that end.
The thing is, a mirrorless camera is pretty much a video camera with a higher-resolution sensor and a slower frame rate. With future action cameras, even that distinction becomes blurred - action stills cameras are getting faster and faster, while video cameras are getting higher and higher resolution. At some point, they meet - probably at 8k's 39MP/25fps. And guess what Canon's spent the past ten years developing?
I can't say whether Canon or Sony will come out on top in the mirrorless era - at present, it looks like Sony is in a better position, but only because they have a rapidly-maturing product line while Canon has yet to show its cards, and the current state of its developments. That could change with Canon's first release. But, no matter how good the Z6/Z7 turn out to be, it's unlikely that Nikon will come out on top, unless you define 'top' in a Leica-like, niche, prestige product sense rather than a commercially successful/technologically superior sense. In the long run, they may do better by designing and making glass for the two big players (and for other optical systems - cars, security systems, robots, etc.), in a similar vein to Sigma or Zeiss, than by making cameras themselves. There's big money in optics - Sony knows this, through its sensor business (photographic, non-phone cameras being a tiny, and shrinking, proportion of their imaging business) - but a sensor needs a lens to go with it, and Nikon makes lenses better than most.