"This mount has been around for so long it's not cool anymore hey can't you make it BOE and let people trade it?" = It must have some sort of value for people to want it enough to ask for such a thing.
They've already watered down the rarity of mounts added from Cata and forward with how they never make them come with Feat of Strengths and rarely add truly unique rare mounts anymore. The idea of making every mount in the game available to anyone the easy way can die in a fire. I'd rather see them visually update iconic rare mounts like the Headless one, Rivendare's, Ashes of Al'ar etc.