The D850 can be said to (almost) have a D500 inside when in DX mode, though it doesn't have the 10 fps of the D500, being only 7 fps (9 fps with a battery grip). If the intended use includes scenes using wider FX lenses, 45mp collects a lot of light, and gives a lot of cropping options whilst retaining a lot of pixels. The sensor has the ability to record a larger dynamic range compared to any DX sensor offering too.
If mainly used as a sport/wildlife camera, with telephoto lenses, a DX camera like the D500 may be a better fit depending on subject distances. Both cameras use the same 153 focus point mechanism, though with the D850, the AF points cover the central area of the scene, with a D500 the focus points cover almost all on the scene. That coverage may be of benefit when tracking a subject, or focusing on a subject towards the edge of the frame.
You do get a lot more pixels with the D850 if you fill the (FX) frame.