Calling WoW stale is a reach. It consistently appears among the top 10 highest grossing games every month. Millions of people still play it and come back every expansion. They have no reason to make a "WoW 2" as long as their subscription base is strong and profitable.

Also, keep in mind that they don't even use subscriptions as a metric anymore, since now they have the in-game store with micro-transactions and the WoW Token which sells for more than the price of a subscription.