All reasonable points. Being a hobbyist just having a good time taking pictures, I actually went from mirrorless back to SLR. I had the more precise focus, EVF, IBIS and pretty good lenses with Fuji.
Then I picked up an old F3 on eBay, again just for fun and out of curiosity, never having touched an F"something".
Then I looked in the viewfinder. Wow!
Frankly, it killed the joy of the EVF for me. Highly subjective, of course.
Lens-wise, well, I find the old Nikkor AI-S primes pretty interesting. So much so that when I had an occasion to touch a D850 in a shop, I mounted the 50mm AI-S on it. Works perfectly. The D850 viewfinder is not at glorious as the F3's but still very, very good and that ground glass gives a pretty positive feedback on manual focus, slightly more decisive than the F3's, even. Since the shop was selling the D850 for a discount, I bought it with the plan to sell my Fuji kit.
Since then I got a few other AI-S primes and 3 modern AF-S zooms.
I have no plan to go back to mirrorless.
Those AI-S primes, well, they are good, much better than one would believe reading all the hype on the latest and greatest lenses. I knew they were great on film. The surprise is they are good on a D850 too. They knew how to design prime lenses 40 years ago and they really knew what they were doing in choosing their compromises.
Zooms are another matter, there I did not consider AI-S, nor even AF-D. The AF-S 16-35mm, 24-120mm (I know) and 70-200mm fl are absolutely more than good enough for me . Having even more resolution in certain cases will do nothing for me. I don't print large enough for one thing. And I don't see the Z lenses being more compact nor lighter than the DSLR counterparts.
In AF, they all focus well enough for me. In manual focus I find I have a much easier time with the D850 than with the Fuji and its EVF.
I don't need the 45MPix, really. I do enjoy the other aspects of the sensor, like DR, colour subtleties and low noise.
I enjoy the OVF, the mechanical soft shutter and mirror noises, all very well damped but giving some life to it all. I enjoy the shape of the camera, the worry-free battery, the excellent light meter, the multi-format, etc... I love than I can share excellent prime lenses between film and digital.
So, I still don't think mirrorless will be everyone preference, and the original question was about a D850 DSLR successor. I don't see any need. I don't see what they could do, really.