the 2 sports are different
in rugby there is "resting" in all the stoppages for lineouts, kicks, scrums
in nfl if there werent alloted breaks, no human would cope, its very fast, dynamic
in rugby no high tackling, in nfl they drive right through the player, they need protection
much like rugby would suffer(mobility/skillwise) from padding, so would nfl suffer from no padding (2-3deaths per match plus disabilities)