I think getting legendaries, even old ones, is already enough of a hassle to prevent excessive use of it for transmog. Furthermore, if lots of people start using them for that, I bet a good number of players that have legendary weapons would simply pick something else out of desire to look different.
I for one have leg daggers, but I'll be damned if I ever use those ugly fishes for transmog. I'd rather use my Wub's Cursed Hexblade that I've been saving for years now.