Funnily enough they already had a solution for that problem - add cosmetic only transmog weapons to bosses. They already did with the Maiden hammer and Taeshalach.Problem with the Legion Artifact system, not only lore-wise being OP, but as well invites for lazy development as weapons can be ignored. And you are right, the problem is discontinuing, just like the problem is that it is most of the time attached to an item. Covenants worked great here, as it was attached to you, the player.
And covenants were better, but still had the feel of "ok, I'm doing all this stuff just for them to take most of it away once 10.0 hits", which still sucks. We still don't even know what will happen to our Soulshape collection once the pre-patch hits ...