Guys are already playing as fast as they can and many teams are teaching SOME form of the Seattle tackling technique if for no other reason than guys with concussion can't play (the gamer quote is "dead toons do no dps"). As well, the league will continue to go hard on leading with the head even if this helmet outperforms and is seen as an innovation in preventing concussions.
Why? Well, the league has to stop the bleeding at the youth and high school level. College and the NFL make bank...now, but fewer and fewer kids are signing up for youth football.
Not that this helmet changes that, but what it can potentially do is reduce concussions enough that especially moms will let their child play...presuming they have this helmet.
Now, if you mean no helmet can be absolutely safe, I would agree with you, but this helmet is even safer than motorcycle helmets in that so much of those impacts may save the brain only to break the neck due to various forces at high speed (and they are generally pretty heavy and neck muscles aren't the strongest muscle even on strong guys).
Tl;dr When I look at the physics of this helmet, I'm genuinely impressed. On an engineering level, this helmet really is as big a difference from the Riddells of the world as hard shell helmets were to leather helmets. If they had this helmet when my son wanted to play football, (he and I have something in common in that we both have brain lesions, me from a seizure, him from a massive car accident when he was a baby) I would have let him play.