tbh I think the answer to the question 'why couldn't you do that 100 years ago?' will be answered the first time they're threatened by like two whole bokoblins and Link heroically steps in front of Zelda, squares up, and proceeds to get the shit kicked out of him until Zelda chases them off with an on-fire tree branch. Link has nearly infinite reserves of power but he can only tap into them when he's not got his attention divided by having to protect someone and can fully focus on becoming the Avatar of Destruction he truly is.