[davidmanze]

Allow me to disagree Most sports shooters do indeed use full-frame cameras already, the lower resolution kind, though (Canon 1Dx, Nikon D5/D6), because they don't need the high resolution but want the cameras to be as tough as possible.

   The D500 is so popular among bird shooters because it combines the fast AF of the bigger bodies with wide AF coverage in the frame and high frame rates. Plus, it adds some extra reach due to the crop factor.

One thing all those use cases have in common: if you have the funds for one (or maybe even more) of the 'big guns', resolution power of the lens will not be the limiting factor, regardless of what camera you use.

 
Not to forget that 200 RAW image buffer btw


........ given your optical knowledge Markus ....... how many Mps can these big gun 600mm F4 (latest models) be expected to resolve on a FF format ??