>Professional football players and boxers have a 100% chance of suffering traumatic brain injury during their careers (literally--a study of the brains of deceased football players showed that all of them had CTE).

We really can't consider this proven. (Admittedly this is a technical quibble.) The players who were studied had some reason to fear they they had CTE. The NFL has done a pretty remarkable job of deflecting both science and common sense from this issue. (Although it's common sense from their point of view to emulate the tobacco companies and try to avoid paying damages to the people they've injured. They apparently didn't look at the end game in the tobacco case.)

It's also hard, looking at the chain of causality, to figure out how the chance might be less than 100%. But we really haven't showed empirically, yet, that the probability is 100%.