For me I use a D810, at this time my D810 is a mirrorless camera, currently getting a new mirror and shutter replacement at the repair shop. I am getting it repaired because I really like the camera, it does have sufficient pixels and gets me all the images I desire.
There is no doubt that the Z8 would give better results as I do take a lot of birds. I guess it is really a matter of cost, with the Z8 I would never use 8K video and could not justify the extra cost but this will change in the future and I may be attracted to a Z7III eventually.
I do see a big advantage in moving to the Z mirrorless as the Z lenses do appear to be so good.
www.flickr.com/photos/14042476@N08/