So unlocking all heirlooms with a single character is "convenience"? Unlocking a golden (light blue in Legion) two-handed mace with a mage is A-OK, but unlocking a purple two-handed mace with a mage is HERESY, HE CAN'T EQUIP IT?
Yeah, no, it's about consistency. Did the player's class matter when obtaining the item? No? Then it goes into the collection so that whatever character can use it in the end uses it. That's the sensible way. It's how an account wide system should work (and it's how ALREADY works, no need to create another, more clumsy and shortsighted, system).
Just because you want twisted senseless systems just for the sake of jumping a million hoops for to really really super validate stuff you've already validated doesn't mean it's "just convenience".
The new wardrobe system wasn't made just "for saving bag space". The infinite cloud space is what was made for saving bag space. There are other features and the account wide appearance unlocking feature is the one that is badly executed, because it takes in consideration restrictions that didn't matter when getting the item and shouldn't matter when storing it, because they never mattered before.