grydth
Senior Master
You make a decent point.....but boxers and NFL players get brain damage as the result of multiple concussions received over their careers, not as the result of a single event.
Multiple repetitive career long brain injuries aren't the same as a one or two lifetime event uses of a headbutt for self-defense.
I would also concede your points. Research is by far not conclusive as yet, and each individual fighter has to decide this on their own.
Sometimes it is possible, given vicious enough hits and prompt medical diagnosis, to determine specific points in an athlete's career where the damage was inflicted. Other times, I concede, this is not the case.