I think we're talking past each other.
Going from a 24 mp sensor down to a 20 mp sensor, you lose about 530 pixels on the horizontal axis and 350 on the vertical axis.
That doesn't sound like much, but if I crop a 24 mp image down to 20 mp, it looks like this (24 mp on the left, 20 mp on the right)
View attachment 188049
Losing resolution means that an editor has less room to crop while still maintaining the quality necessary for print (for argument's sake, at 300 DPI). When an editor has a problem using an image because they
need to crop it, but there isn't enough resolution to use it at the size they want, in layout, it means they have to use a different image. This could mean the difference between you making the cover of a magazine or not!
All this to say, yeah, it would be better if the 1DX mk. III had a higher resolution sensor, because having one would make the camera more versatile. This is inarguable even if one's own specific usage will not benefit from increased resolution.