Again any hobbyist can run around their house in their armor or do backflips for some youtube video and think everything is fine because they are not showing examples of prolonged activity and it's consequences. This study incorporates finding actual abnormal human breathing patterns while wearing plate armor and monitors and calculates the extra energy expended into real statistics , something a hobbyist or armor expert is not doing or thinking about.
I said, one youtuber who owns armor has worn it several hours in a couple of cases - up to 6 hours or so. Not sitting still. He was fine in it.
And you seem to think that medieval battles required you to fight with a lot of stamina absorption. Simply not the case. Formations were rigid, with rarely any extended 1v1 combat ever happening. At most you'd stand poking someone for a few hours
at max until they or you decided to rout. It's not like you can't catch a break either. No one can fight for hours upon hours without rest, armor or no armor.
Yes, you tire faster in armor. That doesn't make it useless. It protects you against the countless arrows, the countless hits, the countless blows that would otherwise kill you. Its worth a bit of compromise in mobility and stamina, especially when you don't need said mobility and stamina to fight effectively.