Outside of the obvious answer, it likely has a lot to do with Blizzard's shift away from counting subscriber levels as a revenue-generating benchmark. If you look at any of the recent quarterly reports, they're not talking about how much money WoW makes from subscribers; instead, they're talking about hours spent playing the game. I believe that the reason this is important for investors (and Blizzard) as it reinforces the popularity of the brand. The more time people spend playing the game, the more likely they are to purchase merchandise or microtransactions. With Vanilla WoW being a massive timesink as it is, this is likely exactly why Blizzard decided to change their minds.
As long as people are playing WoW (in some official Blizzard-sanctioned capacity), the game is viewed as successful in the eyes of its investors.