I don't think a dime should be diverted from the current game's budget to pay for a 15 year old version of the game. Honestly if it starts to eat into current wow's budget I hope they slap on a fee for the vanilla servers. Idc if people hate the game right now, the last excuse I want to hear them use is 'funding.'
Also, people really overestimate how popular 'Classic' is going to be. It's not going to be a current wow killer by any stretch of the imagination. People will rush to play it for about a month, maybe two then poof. The only people remaining with be the blowhards that sit upon their thrones of nostalgia pretending they are the pinnacle of wow players. No dev working on the current game needs to worry about 'Classic wow' putting their job in jeopardy and anyone who thinks that is beyond foolish.