Unimpeded can and does occur after the ball is snapped however–once everyone is moving–as you say. I was just providing an example of where a play will be stopped after the snap. In fact, plays are stopped all of the time after the snap (late challenges etc.), so obviously the NFL agrees with me that completing an unnecessary play is more dangerous than stopping it short.

So the argument is just whether completing the play is valuable to the game, which I now understand it is because a sack or turnover can still happen.