Not only is the upstroke
Not only is the upstroke nowhere near 50% of the available power, most research shows it is not power at all but merely an unweighting of the rising pedal so that your downstroking foot is not working to raise the upstroking one.
Long before clipless pedals we had clips and straps for the same reason, but those principles were already becoming well understood. Unweighting and precisely duplicatable foot positioning were the real benefits.
BTW, my foot condition that pretty much requires me to remain shoeless whenever possible was a result of genetics, age, and repetative motion in Sidi SPD shoes. It took twenty years of daily cycling in the exact same position to push my hallux limitis to the point of life altering status. Weird thing was aside from roller blades and work, it was the only time I was shod.
A podiatrist I discussed this with believes that had I cycled barefoot all those years I might not have become symptomatic until posibly even my nineties! Since I did crash twice (THE OTHER RIDERS' FAULT!) who knows if I would even know about the condition today.....you know.....if I no longer possessed that now limited digit.....