I'll admit I was one of the biggest detractors of Classic before it was announced for this exact reason. A fractured playerbase would definitely be an issue but I've come to realize the situation is greatly exaggerated on forums like this. The endless Classic vs. Retail debates occur here because of the overlap of people who post on forums and people who currently play on private servers. As a result, there's an extremely minor yet vocal subsection of players posting here who are so completely delusional about Classic that they can't handle the mere implication that playing Blizzard's Classic supports the retail version of the game. (Side note: These are typically the people who somehow think "retailer" is some kind of insult.)
I suspect most people could give a single fuck one way or another. At the end of the day, all Classic does is provide more intrinsic value to the current retail WoW subscription. It's something extra to do when you're bored with the retail game (and best case scenario, possibly vice-versa if you "come back" for Classic).
Classic is -- and always will be -- for people who are interested in playing the retail version of the game. Both communities can (and imo, will) peacefully coexist without being at odds with one another. The manufactured outrage on these forums isn't an indication of how things will play out once it's released.