WoW is is nearing an entire decade of being the #1 MMO out there, it's only logical that subs will start to fall in a game so old. Thing is pretty much every gamer out there has played WoW at some point in their lives, whether they liked it or not. The amount of people Blizzard has to reach out to with the game is slowly becoming smaller and smaller. Yes, 1.3 million players in 3 months is an enormous amount to lose, but still.. you have to admit, over 8 million players still subbed to such an old title is pretty amazing, especially with the drastic increase in quality F2P games now on the market.
If I had to predict anything, I'd say WoW subscriptions will continue to drop until the next expansion, rise a bit, then maybe drop some more depending on how good the next one is. I do think the numbers will stabilise around the 5 million mark though, as that's likely the amount of hardcore with-it-till-the-end WoW fans knocking around (me being one of them).
We are getting to the point in WoW now (or just past actually), where most of the old storylines from the Warcraft series have been resolved, and new ones must be created for longevities sake. Some people don't like that. I however am thoroughly enjoying WoW just as much as I did 5 years ago, perhaps even more, and as long as Blizzard keep throwing out these quality patches and interesting new expansions, they will keep getting my 8.99 a month.