Wait... You have not beat Plantera with a Copper Shortsword (Legendary) before?Maybe. It's a pretty good and balanced idea, but it doesn't make too much sense to turn the most useless item you could ever want in the game into a weapon you could use to beat a hardmode boss.
That's because: Re-Logic.Wrong.
Broken Hero Sword + Copper Shortsword is way easier to make than Broken Hero Sword + Excalibur, and yet the first recipe is more powerful than the second!? This idea is funny and all but does not go with the logic of Terraria.