Some facts.
1. There will be growing pains and people will point to them as proof that the rule is a complete failure. They will be wrong.
2. It has been negligent that they have not implemented this rule, or something very much like it, before. If they care about limiting brain damage, it is a no-brainer.
3. Everything will be fine, and running backs will be able to run with their heads down as long as they aren't using lowering it at the end of the play to use it as a weapon and smash someone.
4. This eliminates a great deal of subjectivity about whether the person getting speared in the head is a defenseless receiver or not. Now they don't have to decide: just throw the flag.
In the meantime, get out your popcorn and watch the macho spasms bemoaning what is happening to football. It is always entertaining with every rule change meant to protect people's brains.