I'm a little curious about the absence of a release mode turret on the Z8. At the risk of beating the "D700/D3 duality" dead horse even more, this seems like it wouldn't have been difficult to incorporate. The release mode button from the Z9 is there in the same shoulder cloverleaf location, so if that didn't become an overburdened control on the Z8, why would the Z9 have needed two controls for one category? It's like the turret was there mostly for the comfort of the D6 et al crowd, and the button is there for adding functionality via firmware updates... and try to not to worry about the overlap.
Aside from that curiosity, I'd like to congratulate Nikon on the wisdom to carry over very nearly all of the physical UI from the Z9, "from the knees up". It will prove to be a welcome layout similarity when using an '8 as a 2nd camera to a '9. I would have dearly loved to have had that in Tanzania a few months ago. My 2nd camera on that trip was a Z7, and even after having done as much as I could to converge the button assignments there was still enough of a difference to have slowed me down on my first shooting day.
I personally know one Z9 owner who will switch over to an '8 but I don't think that anyone's predictions at this point about how often that will happen are based on anything concrete. I may very well pick up an '8 in maybe a year, and I'm pretty sure that I'll eventually sell off the '9 due to weight... when I'm 80 years old 😁. When I get an '8 that may be the time to have my '7 spectrum-converted.