If I recall correctly they were never available transmog like most old school legendaries and they added it as a transmog option because warglaives in weapon type with the lowest amount of items. Add more warglaives before stealing our thunder I say.
Because DHs have very limited mog options when it comes to Warglaives, and it makes more sense that it’s limited to DH imo.
But they should make other legendaries moggable in the same way they made the Warglaives moggable. By having the achievement for obtaining them and finishing their respective timewalking era raid.
Like if you wanna mog Shadowmourne/the one hand mace then kill Yogg-saron. Or if you wanna mog the Rogue Daggers/Firelands staff then kill Ragnaros, and so on. Also they should include classic timewalking with classic raids for classic legendaries.