There is no question this will fundamentally change the game. So running backs, who have been taught from about 8 years old on to lower their pad level now have to take care not to dip their helmet so as to lead with the crown? There is no way the game can be played the way it is today with this rule.

This piece makes a great point about how this rule is fundamentally different from previous rules designed to keep players from using their helmet as a weapon in a catastrophic way:

But this running back rule is targeting something different — necessary violence. The game of football involves thousands of small-scale violent collisions. Until now, a running back crashing into a defender with his head was considered a part of how the game is played. It was an example of the inherent violence of the sport.

"Necessary violence." How can you play running back in the NFL without leading with your helmet at one point or another.