There's seems to be questioning on powering the upstroke, but that has never been "the reason" for foot retention systems. Except for one specific type of cycling-- track cycling, where upstroke IS USED in high acceleration starts like in Match Sprint or short time trials. There are track versions of clipless systems which require a lot more force and twist to get out of.
The main reason for clipless, as I understand it, is for safety and maintaining one's foot on the pedal. When you're pedaling, if you are apply any down-force whatsoever on the upstroke, you're wasting power. So, a trained cyclist will always have their foot practically floating on the upstroke and their foot will drift position as they pedal unless something is keeping it in place.
The main reason for clipless, as I understand it, is for safety and maintaining one's foot on the pedal. When you're pedaling, if you are apply any down-force whatsoever on the upstroke, you're wasting power. So, a trained cyclist will always have their foot practically floating on the upstroke and their foot will drift position as they pedal unless something is keeping it in place.