Yes, I think there are marginal benefits to Nikon to upgrade the D850 but potentially high costs.
The D6 autofocus module and processor has already been developed, and the Z7 / Z7 II sensor with on-sensor PDAF also exists. there is probably not that much cost to making a D850 upgrade by using these components, yet it would significantly enhance the product in both modes of operation. The D6 makes shooting figure skating phenomenally easier due to its custom group-area modes which by the way do not exist on any other Nikon camera, including the Z9. However, for some subjects such as birds the D850's higher pixel density is an advantage. A D850 successor with these improvements would probably stay in the same price class which is about 50% cheaper than the Z9. I.e. a significant advantage to users. And you can be pretty much guaranteed that it won't focus on the background when in custom group area mode, which is more than can be said of Nikon mirrorless which is a pain because e.g. the Z6 II wide-area with eye detection AF mode frequently focuses on background features. Since the D6 AF system is made of cross-type points only (given sufficiently fast lens) it is very reliable in not focusing on background grass whereas the Z6 II with its vertical only PDAF sensors can sometimes focus 95% of images on the background grass (in backlight) and with great pains 5% of images can be focused on the deer, which is extremely unsatisfactory. Z9 is no doubt better but it's in a different price class and it too doesn't feature a single cross-type AF sensor. Not even one. So it always prefers vertical lines over horizontal ones unless there is a subject that it can recognize. In my experience the Z6 II AF is deeply untrustworthy and unsatisfactory experience in general. What it does well is landscape, video, and portraits (where the subject stays still). Additional annoyances include the remote control interface (MC-DC-2 terminal) which gets boggled up with the use of L-brackets and the remote cord has a very poor tactile response compared to MC-30A.
IMO there is a good opportunity for Nikon to sell a D850 successor (with D6 viewfinder AF and Z7 II LV / video AF) in quantity.
If Nikon could make a Z8 or Z7 III in the D850 price class and put Z9 AF into it, that would be great. But they will not. That kind of AF commands now 6300-8000€ price in a camera body (depending on brand). So, if one wants a Nikon that can focus on a moving subject with reliability, that's what one has to pay. Unless of course going with a DSLR.
