It is the repeated trauma that is the problem. But mostly it's repeated concussions, which don't happen on every tackle, even every tackle that hits the head. If you move your head, you aren't put at risk, and what a good helmet does is make it so that instead of banging your head, you move your head. big difference. As long as heads are not targeted, there should be a significant lower risk factor in the sport. Accidents will still happen, but they happen in every aspect of life. You can't prevent everything, and making it so that even accidental contact and hits are punished means that no one will even try to tackle in the upper body, and even then it will still happen. So now you have the problem still occurring AND people taking out knees.