Not surprised. Even with the availability of more difficult content (heroic raids, challenge modes, etc.) there seems to be fewer people willing to do that difficult content. And without people willing to do the difficult content, the glue that holds guilds together disappears. Instead most people would rather do LFR and complain about how it is too easy, and the unsubscribe when they get bored.
BUT, the game is nearly 9 years old. If I had a 9-year-old game that still had 8.3 million subscribers willing to pay the monthly fee, I'd be ecstatic.