Idk, I think having un-obtainable items being left alone is a better idea, kind of like bragging rights to those who took the time to earn them (if it had a 0.01% drop-rate or is not available anymore). It saps away the collection/rarity aspect if say 50,000 additional people can now have it from paying for twitch subscriptions or watching a stream. They realistically should just create a new mount to release with this expansion, and perhaps have it be interactable like the expedition yak.
Also, being able to sell your account or the mount for $3,500 would be amazing, totally against ToS but worth it for the owner. Just bank $3,400 and spend the other $100 on a subscription & level booster, assuming all goes well.