Unpaid college and highschool athletes and CTE are two separate issues, but that's why I chose the discussion title I did.
Another big difference between rugby and american football is that in rugby, being tackled doesn't stop the play. All it does is stop the forward momentum for a while, and then it can continue when a team mate picks up the ball from back down the field from where it was stopped.
The incentive for the rugby player being tackled isn't to keep moving forward at all costs, nor is it the tackling player's goal to stop the ball moving forward at all costs. So in american football, as soon as a tackle begins, the best way to ensure it is successful is for as many other players to join in! All the defending players pile on, and all thee attacking players get behind their guy, often charging in at high speed.
Of course, because the play doesn't end with rugby, anyone pilling in or laying around on the ground is just a liability when the play gets going again.
My proposed fix for american football is to stop play with a successful tackle, but the ball is then placed on the grid at the point the ball was at the first moment of contact of the tackle that was later successful.
So, if a player is has the ball and is heading towards a player from the other team, they would then want to:
A. Avoid contact at all costs.
B. If they can't avoid contact, try to get away from the defending player cleanly.
C. If they think they can't keep get away, drop to the floor as soon as possible. Any extra inches the ball moves forward after the initial contact is a waste of effort.
For the defending player making a tackle, they are then aiming just to take the player with the ball DOWN, not stopping them from moving forward. This means lower tackles would be more successful. The best tackle would be to grab hold around the waist, and try to add more and more drag to the player. As long as you don't lose contact, the ball will be reset to the point you first grabbed on.
This isn't a perfect solution, but it would maintain so much of what I like about the game, but limit the (to me) most violent part of the game, where a player is upright and five other players ram into him, aiming for the highest possible impact. No thanks!