Helmets don't solve (at least presently) for concussion and concussion flow on effects. In fact, some scientists believe they exacerbate the issue i.e. the brain moves or gets rocked around inside the skull easier with a helmet. Think like a person gets thrown around inside the shell of a car when they have an accident. Many believe it only helps where external injury to the head is concerned and even then, things like fractured cheekbones, jaws etc can still occur.
How long have they been wearing helmets for in the NFL vis-v the huge concussion and CTE issues they have ?
As someone who regularly cycles, and opposes bicycle helmet laws as we have and only 2 other countries have, this is very much the case.
In cycling the helmet protects from things like a few stitches to the head when you hit the ground, but there is no evidence it helps with concussion or similar injuries at all. The problem is that when you hit your head it is the brain sloshing around inside the head which causes concussion, not the impact per se. A helmet has no effect on the brain sloshing inside the head when head meets immovable object. As The General points out above, the improvement in cars was the crumple zones, the car stops very quickly which can cause a lot of issues inside the head, but spreading the halt of the car by having the car crumple, and therefore not immediately halting the occupant of the car, seems to make a difference.
Even bike helmets attempt to crumple (hence the foam, but if the foam breaks then the helmet has failed) but you see the size of those things in terms of the thickness around the skull, and anything which actually slows down the way a head stops when it meets the ground at speed would be way too unwieldy for a footballer. Not sure what the NFL helmets are like but they have huge concussion issues so no solution there. I can see little point in the padded helmets some wear, although we don't know enough to be definitive, they look to have little of no impact on the brain sloshing around in the head when head meets immovable object.
This is not an easy issue to deal with.
DS