That said, is anybody actually paying these prices for an 830 now?
The cheapest prices I've seen for an excellent condition D850 is around £2000, they have gone up recently. There are very few excellent condition ones cropping up, and they seem to vanish as quickly as they appear, so yes people are paying these prices for a D850.
If you are a Nikon user, sensor wise, the nearest mirrorless equivalents are the Z7II and the Z9, but the Z7II s/h will cost you another £1000, and the Z9, several thousands of pounds more, assuming you can find one s/h or even new.
Comparisons I've read or watched suggest that the D850 image quality still has a slight edge over both the Z7II and Z9, not enough to convince me, on its own, to buy a D850 over a Z model, but enough to convince me there is unlikely to be any image quality benefits from spending the extra money on a Z model.
Ignoring the Z9 for now, the D850 is still a better multi-purpose camera than the Z7II, with its higher fps (especially with the battery grip) and buffer making it a better wildlife/sports camera, while still giving the high resolution that might be desired for landscape and the selective cropping often useful in bird photography.
The Nikon D850, has Nikon's professional body design (as does the D500 and D6) which has, in the past provided much better handling than the pro/enthusiast models (e.g.Nikon D750), and only the Z9, so far, offers this professional body design option.
For me, as a Landscape and Wildlife Photographer, who cant afford a Nikon Z9, the D850 is still the obvious buy as an update to my D750, and a companion to my D500. My existing Nikon F lenses, and any I buy, will still work on a future Z mount camera via an adaptor. Indeed in some instances lenses like the 500pf are reported to work better on the Z9 with adaptor, than they did on native F-mount bodies.
Of course there are some nice to have features with a new Z body, but in comparison with a D850 (ignoring the Z9), if I bought a Nikon Z7II, I would be spending a lot more money to end up with a less capable camera.
This would not be the case for everyone of course, but for someone with my mix of interests, my budget, and an existing Nikon user heavily invested in the system, a Nikon D850 is still the camera to go for.