I agree. There is one glaring draw back of the Nikon Z9, and this is the reason I have, and will not purchase it. And I have used Nikon's since 1969.
The Z9 will loose track on a moving subject, your D500, using GROUP AUTO FOCUS, will not loose tracking on a flying, moving subject, especially if it gets lost in the background.
Why Nikon choose to drop GROUP AUTO FOCUS is beyond me, but they did.
So, if your thinking about a new Nikon purchase, and you like to shoot birds in flight like I do, I would strongly suggest going to Sony, like I did.
It was very hard saying good bye to my beloved Nikon line of camera's, I carried them into tough situations while shooting for UPI (united press international) and as a professional wedding photographer for over 35 years.
But Nikon just keeps messing up, I thought their Z9 would bring them back, but they have had issues with tracking since it came out.
I started using Sony with the a9 about two years ago. This year I am selling off the rest of my Nikon line. I will retain my Nikon D3s and the 60 and 105 micro lenses, my Nikon SB900 flash, and the Nikon 24-120 f4. All of the other equipment will be going and I will switch completely by this November with the purchase of the Sony a1.
It is a sad day indeed, but I have written Nikon, watched there blogs, and have not been impressed with their responses to me, and I have been a member of Nikon Professional Services since day one.
So, my advise is, if you looking to go mirrorless, is to do what I did, or go Canon, they seem to be doing a good job with mirrorless bodies also.
Good luck and keep on shooting until the end.