It seems weird to me that people would go out of their way to dislike something that doesn't affect them in the slightest. The only way vanilla WoW could prove to be a threat to retail is when its getting popular enough that retail may lose a lot of players, which could hinder their guild progression or something. But if your guildies will move on to vanilla servers full time then they might've not liked playing retail that much after all. These servers have not taken any resources from retail yet still people continue disliking these servers. If your afraid of vanilla's succes, then you could rather try to make retail better instead of trying to take down its possible competitor. Give proper feedback to the devs like the vanilla community is going to do as of now. There will be a lot of different suggestions, but thats up to Blizzard to brainstorm and to find out what the best approach for their game is.
Vanilla compared to Legion (soon to be TBA) is a whole different game. And I expect people to play them for different reasons, which will result into healthy playerbases for both versions.