Mmmm I dunno. We do know that the left has been attacking football from within, as it is a uniquely American, and macho sport. However it is so ingrained into American culture and tradition that I can see it taking a hit but still surviving.
So it would be a win-win for the left. If they remove the anthem and the NFL goes under, they win. If they remove the anthem and the majority of fans, however slim, accepts it (grudgingly or otherwise), they still win.