Do you think the fact that Warmane is successful is due only to the fact that it has pop and is wotlk based? You have to understand why Warmane is successful to be able to anticipate how Blizzard wrath would affect it. A couple things (beyond the simple fact that there is a community here that many people embrace and many others have sunk a lot of money into). First, many people won't care for normal xp rates that Blizzard would provide, they are used to faster leveling of characters, professions, etc and would find 1X less appealing (save those who play on Lord). Also, Warmane remains free to play, and that is always a huge plus for many people who play private servers. Finally, Warmane is an international community, this is something big biz like Blizzard wouldn't capture as they would have regional servers. I cannot understate how huge it is to the community that you get to experience many different cultures and languages and meet so many people from all over the world, this is incredibly special and I don't think it is as fully realized or appreciated but it is a staying factor for sure (as someone who moved half way across the world due to friends made in this game I feel strongly about this).
TL:DR Warmane should be just fine, and I don't anticipate Blizzard threatening their numbers too much with a "retail release".