"If an army in full plate can defeat a force > 2x its size, it would seem economically viable to cut it in half and armor it." You cannot really generalize tho

"If an army in full plate can defeat a force > 2x its size, it would seem economically viable to cut it in half and armor it." You cannot really generalize those things. It depends on the battle site. Heavy cavalry cannot operate everywhere. Not in a city, not in dense forest, nor mountains. And not in heat. In general, armor limits mobility. Also the heavy cavalry is an elite formation, taking years to train the knights and (breed) the special horses. Hard to scale. A simple human infanteryman on the other hand: give him a cheap mass produced rifle, drill him some days and then put him into a formation. That scales easily. Then you can also give him a long lance and drill him some days more and any cavalry charging at a prepared position will be a bloodbath for the horses. And once down, the shiny knight could barely move and was an easy target. But yes, at the right moment, a heavy cavalry charge at cheap musketeers was indeed shocking and routing formations en masse. It takes great discipline holding a line, when thousands amored battle horses charge at you.