It is interesting watching the D6 in action in light of where we are mid 2022. This was recent - late 2020. Not that much has changed
Obviously, besides the camera a great deal of success with capturing challenging subjects (including action) depends on the photographer's abilities.
For a long time i dominated getting brilliant special money shots in high end surfing photography, it helps when you have excellent subject materiel and conditions, this often sets you worlds apart as dose perfect wild life action opportunities. the smaller part of the equation is the glass and camera and how you use it.
In a nut shell, the D6 is super sticky in challenging sports action situations and frankly i have seen it stick in these conditions better that the Z9 and even the A1.
Bottom line is the difference between 14 or 20 fps is not a game changer, 80 -90% of the end result comes from you, and if you have stunning subjects and conditions well say no more......
I think the D6 despite others take on the subject is a very sticky camera..........and in very challenging sports action conditions is ahead or mirror less still, now i know some will disagree and that's perfectly fine.
I think mirror less is moving in the right direction but defiantly needs more time............
I think the Z8 and Z9 will be the new and best bench mark - combo for Nikon.
Only an opinion