There are more concussions now than in the past (of course in the past we didn't know what we know now about concussions). Players are bigger, stronger, and faster. The big hits come in the passing game. Physics takes over from there.
I'm not saying guys now are tougher or anything, or even that it's more physical, but the game is still extremely violent.
Bears are most likely one and done. Especially if they end up playing GB or NO.