Why would not having helmets decrease concussions? I've seen this arguement tossed around, but never really justified. I'm assuming the theory goes that if you aren't wearing a helmet, you won't lead with your head. This ignores a couple of things.
1) Normal people won't lead with their head. NFL players are fucking nutjobs. Hell, a ton of small-school college kids are fucking nutjobs.
2) There is a ton of shots to the face involved blocking/sheddling blocks. Let's say a fullback is coming up to lead block a linebacker, that linebacker is coming into with a forearm shiver to the face.
3) Guys end up on the ground, a lot. That's a ton of knees to the head that have nothing to do with tackling.
The only way to flat out eliminate concussions is to eliminate football. Otherwise, enforce the ruels about leading with your head (spearing), and use these mouthguards that are designed to absorb the shock (keeping it off your brain).