Why should I switch from my Nikon D800/D500 to a Z9?
What will I gain?
You will probably not gain any experience, knowledge, or technical skill.
You will not gain sharpness, that is, if your skill is at least good enough to take a sharp image. The Z9 will not make you a better photographer than you already are.
You will gain 10 to 13 frames per second if that matters to you.
You will not have to FINE TUNE any more lenses, if you ever did.
You will be able to, with the Nikon adapter, use your current Nikon lenses.
For me, the biggest advantage is that I can see exactly what my image looks like (exposure wise) before I take the shot.
You will gain little in the way of assigning buttons for various functions so you can keep your eye on the subject, but if you do not assign camera functions to your buttons you will not gain much.
You will gain a drain in your savings.
You will gain silence with your new shutter, you know how noisy your D800 is, your D500 is not that much quieter. So you won't scare as many birds or grandchildren when you trip your shutter.
You gain a vertical grip and shutter button, you will not have to buy a separate vertical grip.
You will gain a very sturdy platform to mount your long telephoto lenses, you will gain balance with your long lenses.
If non of these things matter to you then I guess you have answered your own question.