I wouldn’t advocate pulling heavy sleds all the time for anyone. But what things, other than contact time, are altered when pulling a 20%bw sled vs a 10%bw sled? Keep in mind I’m talking about the first 2-3 explosive steps which are critical in many sports. Also, I remember some of the hills in CF’s GPP that looked steep enough that they would slow the athlete more than 10%.
Posture and mechanics.
Football is about applying force quickly, at some point, you will be slowing the athlete down with too much weight as well. I wouldn’t think you would want to advocate HUGE pushes for a footballer anyway because they need to be able to cut. So they would need to actually alter mechanics in order to stay grounded. Football is a different than track in many aspects for that reason.