IMO the percentage of mental type injuries among all players is very small, tiny - better to outlaw driving cars. (sarc)
It's difficult to say, because increased awareness is an element (quite possibly a large factor) in this.
I've heard enough people who know that the players are bigger, faster, stronger than ever before, and the pads and helmets turn the entire body into an airborne weapon.
I'd like to get this turned around before someone dies on the field (as a result of a hit). 2 years ago, Eagles MLB Stewart Bradley took such a shot to the head that he literally staggered towards the wrong sideline, and was back on the field two plays later (I believe he was then benched the rest of the game).
One of the big issues is that every player down to the lowest levels (grade school) have been, and are taught to hit high. How do you suddenly change the game back to where it was in the 60's and 70's? I guess that's why Roger Goodell and his ilk get paid 20 million a year...