WoW is probably not more than 5 million when you throw out China. While there's nothing that says only a sub from Europe/NA counts as a sub, the rate they charge in China is very different too so it is not exactly a fair comparison. As far as I can tell WoW is closer to a F2P in China than a traditional MMORPG even though it's not exactly free. FreeRealms has 10 million subscribers so it's certainly a lot easier to pile up massive numbers in that model.
But even at 5 million that was still well above what anyone thought WoW could've pulled, including Blizzard themselves. WoW also made quite a few mistakes and didn't take off until sometime close to WotLK, and it was helped by the fact that some guys like EQ2 or Vanguard flopped despite having arguably better talent, probably because those games are headed by guys thinking people like to spend 200 hours of time doing nothing. Since Blizzard stopped releasing numbers after WoW's numbers stopped growing, it could potentially declined by a lot and since there is no law requiring people to disclose their numbers, WoW's subs is going to be 'more than 11 million' for the forseeable future because they're not obliged to tell you if it ever went down.
The problem with WoW is that it is ridiuclous easy to copy its style and WoW's graphics sucks so once people stopped trying to be the anti-WoW it's really easy to out-WoW in its own game. I think EQ2 can easily do this and it's increasing numbers recently seem to suggest SoE is just as good at doing this, if not better. I don't think you'll see a single unifying game beating WoW on a global level especially given Blizzard is not obliged to tell you the declines, but it wouldn't surprise me if 2 years later the highest revenue generating MMORPG in North America is FreeRealms or Runescape or some F2P you've never heard of, and the top MMORPG in Korea might be some RMT-based FPS, and top revenue generating game in China could be some web-browser based stuff.
I recall one of the Activision guys was saying you can't beat WoW even if you have a billion dollar and to me that seems more like they're worried and trying to scare people off. Certainly if you look at games like Champions Online and Star Trek Online it's almost looking like there's a MMORPG generator you can use out there. Sure it's not going to be the WoW-killer but it also can't possibly be very expensive to produce a generic MMORPG. Look at the screenshots of some generic F2P MMORPG you've never heard of, they all look like they're generated by the same engine and while there surely isn't a MMORPG maker, it's pretty obvious that most of them run on similar concepts so you don't need to reinvent the wheel either.