I don't think this comparison stands up, at least not for most reasonable interpretations of "aggressive skiing". It might not involve any crashes at all.

I think there's a fair distinction to be drawn between sports that might cause head trauma if you make a mistake, and sports in which head trauma is practically the defining characteristic (see also: boxing). I am three weeks away from having a son, and while I've got some years, I'll eventually have to make some decisions about what activities I want to encourage and discourage. It at least seems possible to learn to ski without repeatedly bashing your head. Not so for football.