That Z9 will be yesterdays tech in a year or two, and you'll be able to pick it up for a fraction of its current price. It'll still perform exactly as well as it does today, but the gear heads will be onto the next new thing.
It probably won't see significant price drops for 5+ years, because the pace of development in the DSLR market is slowing down. There are longer and longer gaps between new significant features, which means the price of used cameras isn't going down as fast as they used to.
As frustrating as it is, I must say it is kinda nice to finally have one sphere of consumer electronics where you don't have to update all the time!
I've never bought flagship gear, and I often buy second hand. This is the way.
Because why would a camera and a lense, that was the top of the line back the day, be less suitable today? Not that a Z9 wouldn't be nice, but I have other hobbies as well and want to see places!