I agree that legendary weapons should have infinite durability. When playing through Blazing Sword the first time I recognized that the story was building up to me collecting the legendary weapons and I naturally assumed they would be indestructible. I figured that, along with some advantage over a specific enemy, would be their main reasons to exist. I can understand some of the named weapons having limited durability but the ones that are legendary national treasures that have survived centuries shouldn't have the same weakness. I get that it's for balance, but as you said, if they're received near the end of the game there's especially no reason for it. Now that I think about it, if balance is the issue a new mechanic similar to the Master Sword in The Legend of Zelda: Breath of the Wild might work. Legendary weapons could be strong and unbreakable, but their effectiveness could go on a cooldown at some point to prevent them from being overused. On the other hand, there could also be a certain condition that needs to be met to utilize the weapon's full potential. I'm not wild about durability going away completely, but I think this would be a nice way to balance things.