Yeah, I kept my D850, too. It's such a fantastic camera, I'm never giving it up! (Plus, I use it for film scanning.) Problem is, I now have enough Z glass that it becomes a bit unwieldy to carry both sets and the Z glass is often far better than my aging F glass. So I'm hanging onto the Z6II as a Z-based backup for trips where baggage weight is a concern, at least for a while. I totally feel like I'm in backup camera limbo, though.
If the Z8 doesn't meet my expectations, I may have have to put serious consideration into a second Z9 down the line, much though I'd rather have a smaller form factor for my backup.