The question is the price point.
I recently sold my D850 to add the Z6II at my kit, I already have the Z6 and a D5.
As things currently stand, given the small difference between the two Zs, I regret it a little, but I had to lighten the kit and start the transition to mirrorless.
I think my situation is quite common to many, several friends are taking the same step. The Z9 will only be welcome if it is aligned with the D850's release price, but it will hardly be the case.
For my kind of photography, wildlife and landscape, the 850 was an excellent camera but it is not a multipurpose one.
In fact, I bought it to complement the D5.
A Z9 at the price of, for example, of a D5 I don't think it will make big numbers in sales. At least, I will hardly take it into consideration, especially if it will have the same sensor as the previous one as it could be seen that the resolution probably is the same. The only advantage in the new release is that it will probably lower the price of the Z7IIs that I am waiting to buy because in my opinion, to date, they are not worth the money, at that point my D850 was better. My opinion, obviously.
Here is Oz our club members are sitting on the D850 and D750 and waiting for the mirrorless fruit to mature more, also the fact that there are so many more changes on the horizon.
The D850 and D5 are the ticket for many as it covers all basis, if weight is an issue then there into the PF glass at all levels.
Quite a few have taken up the Z6II and pairing it up with the D850, you see many don't want the weight size and noise of the D5 as there are a lot of bird specialist shooters in the club.
We will never get a camera that dose it all LOL, its always we need 2 or 3 cameras, if you don't want the weight and cost the only solution is compromise in what we do.
Ok the D850 and Z6 II is popular, some have gone fully Z6 II Z7 II, my self I sold my D4s D5 with absolutely no regrets and got 2 x D850 bodies a nd a 70-200 FL.
The one d850 has a power pack perminatly, but usually I only ever go out with one camera and one lens well mosty.
Honestly everyone is different, but myself I find there really isn't anything I cant do on the D850 with the right glass.
I had been lent from Sony an a A9 when it came out, wow 20 fps, I needed to wind it back to 10 fps, I mean if you cant get the shot in 10 fps your the problem or just video it.
Resolution colour are my preferences, the bonus is a little speed, if a D850 replacement came out with 12 FPS at 45 MP and didn't do less than the D850 in other features it would be all I want, why do you need two cameras, simple, because they up the speed of the new Pro D6 equivalent, we will always as consumers want what we cant have and that's the one camera that dose it all.
Nikon needs to focus on its customer needs at is sorely lacking.
Be at peace, Oz down under