Long term sustainability -- gone. WoW lives on new blood, not old. (SRC: quote about more people who quit than currently play WoW by
BLUE)
Long term growth -- gone.. WoW is in decline. New expansions give some oxygen, but overall it's not going to be as good as 3 months ago, or 6 months, or 9, or 3 years ago, etc.. Alive? Yes. Growing? No.
Having Legacy might not be just pleasing a few hundred thousand fans. To many, Vanilla WoW will be new, and may be enticing to many who see it as a new game (some on Nostalrius saw it as new too btw).
It really depends if Blizzard opens Legacy servers for paying accounts, or they just legally off it to Nostalrius. Honestly it would be awesome if they just open it in-house for a $12-$15 WoW account, IMO. What they do, who knows. It sure would be nice though.