It depends in large part on how close you can get to your subjects, how large they tend to be, and how much you want to invest in super telephoto lenses. A D500 provides an image size with a 400mm lens that with the D5 would require a 500mm lens to achieve.
The D5 in DX mode becomes a 8.9MP camera with less resolution than the D2x camera. Photographing distant wildlife, as with grizzlies in Yellowstone, or small to medium size birds in most places and one can use the 800mm f/5.6 with the D5 or the 600mm f/4 with the D500 (or D850) and get the same image size. The longer focal length lens also results in more perspective compression and less DOF at any given aperture.
When I tested a D500, D5, and D850 while photographing osprey returning to their nest I found that only in one situation was the D5 clearly superior to the other two cameras in terms of its autofocus performance. That one situation was when the bird was quickly coming directly toward the camera, which is the most difficult situation for any autofocus system. If I was shooting sports events then the D5 would be my choice as there are no "re-do's" in sports. This is not the situation with regard to 99% of wildlife photography.
Photographing from a boat as with trips I made last year to the Pantanal and to Costa Rica, the D850 was the ideal camera along with the 80-400mm and 500mm PF lenses. I was photographing birds and snakes and frogs and lizards at distances often of 150 feet. No way to fill the frame and a D5's 20MP image file would have required cropping and not provided the best picture quality. When I returned from Costa Rica I sold my D500 and bought a second D850.
If the temptation is too great and you buy a D5, then be sure to buy one with the XQD card slots and not one with CF card slots.
