Nearly every year the NFL announces tweaks to the rules designed to protect players and improve the game. Face-masking, clothesline tackles, and spearing with the helmet, once perfectly legal, are now considered dangerous relics, and it’s hard to imagine even the most hidebound fans wanting them back. ... Violence will always be endemic to football, but a page from history’s playbook demonstrates that making the sport kinder and gentler—even dramatically overhauling its rules—won’t necessarily make it less appealing. In fact, it could ensure that the king of American sports continues to enjoy a long reign.