Read and react doesn't mean players don't have assignments. You still have to maintain lane integrity and run fits. Just because all 11 defenders are trying to stop the run doesn't mean everyone sprints will-nilly trying to bring down the ball carrier. There is always someone responsible for gaps, players who's job it is to prevent a cutback, players who take on the lead blocker, and players for outside contain. "Read and react" does not mean "make the tackle at all costs".