Here is a list of current Nikon lenses which are compatible with the FTZ Adapter. For older Nikon lenses there may be a list somewhere, as they were claiming specific numbers were compatible, so you would think there would be a list.

For third party lenses, in theory they should work, but until the cameras and the adapter is out in the wild no one will know.
Well some people may only have DX lenses. Not everyone can buy everything at once, as you say they are expensive cameras.
The benefit of using DX lenses on a mirrorless Nikon camera is that the viewfinder should be filled rather than looking at a cropped view with a lot of the viewfinder shaded out on a FF DSLR. Use a DX lens on a Z7 and you would still get 20Mp. With the Z6 you will only get 10Mp with a DX lens.