There's a thread on a different game forum about a bunch of private servers running vanilla. I don't play myself, but I've checked it out from time to time, and it seems that people in that thread loved it for a little while, then got bored. I think we can assume that there isn't a market for legacy realms. But I guess that Blizzard could keep maybe a couple of realms decently populated, but I do think it requires too much administration for them to implement them. Maybe they aren't really compatible with battle.net anymore, I don't know...