Also, the equilibrium is technically in yards per play (or expected points per play, probably better) and NOT in the rank. This is important since passing is generally better than rushing for the offense, so a defense could rank worse in rushing defense but still want to focus on pass defense as that yields more exp points per play.

I don’t know about college, but there were rule changes in 1932 and 1955 that redefined what it meant to be a down runner. These were both for safety, as it was common to just light up ball carriers when they were on the ground.