there was a total of 100 million paid unique subscribers during it's tenure. meaning that some played for months, some for years and there was a certain rotation. Also, there is the warcraft 1, 2 and 3 franchise that drew millions as well as Hearthstone that is based on warcraft that has 30 million active users. Just because someone stopped playing the game, doesn't mean that they don't want to see the movie.
As far as the 5.5M subscribers, think about this, the game has been out for over 12 years and still has a freaking 5.5M subscribers! people were still using playstation 2 when WoW came out! ps3 didn't come out yet! tell me a game that came out at that time they have that many active players(let alone paying players)