MMO-Champion mentions interviews the Blizzard developers gave on patch 4.3, in which they made a side-remark that "There are more people that played World of Warcraft but no longer play World of Warcraft than currently play World of Warcraft". I believe that this has been true for years already, and that the number of ex-players is not just "more", but significantly more than current players. The reason I believe that is that I regularly stumble upon PC games sales charts, and World of Warcraft has been in the top 10 for years, and pops up again frequently whenever there isn't a great number of big new releases. Thus if sales of World of Warcraft are strong over years, but the overall number of players has plateaued and is now declining, it means that as many or more players leave WoW.
I don't know if there are 15 million ex-players, 20 million, or 30 million. But as Blizzard themselves said, it is more than 11 million, and that by itself makes the number of ex-players huge. And that is very important, because most bloggers and commenters tend to forget about these ex-players when discussing size of the market. How often have you heard arguments about some new game like SWTOR "hurting" (if not "killing") WoW, because "the players of the new game have to come from somewhere".
Surprise, surprise, that is simply not true. Even if the analysts are right and SWTOR will get 3 million subscribers, that does NOT mean that WoW or other MMORPGs will lose 3 million subscribers. Because the overall market size is NOT the sum of all subscribers of all MMORPGs, but is in fact a much bigger number. It is likely that for many other games the same statement is true as for WoW, and there are more ex-players than players. And not all ex-players of game A are now playing game B. For example I am not currently subscribed to ANY monthly subscription MMORPG, nor do I actively play any MMORPG in Free2Play mode. Nevertheless I'll certainly pick up SWTOR and Guild Wars 2, so these games can obviously gain subscribers without another game losing any.
It has become pretty much impossible to track all the potential players of MMORPGs. People take breaks from MMORPGs, play single-player games, play social or browser games, or play Free2Play games where they appear as "customers" regardless of whether they are actively playing or not. Even if you only count North America and Europe, there are probably well over 20 million people who at some point in their live played some variation of online role-playing game and are likely to do so again if something sufficiently attractive comes along. A big new game like SWTOR is able to attract players from this ex-players pool, and significantly grow the number of "active" MMORPG players.
That is not to say that new games don't also attract players currently playing other games. And given its market share, a lot of players switching games will come from World of Warcraft. But don't think that whatever number of people SWTOR attracts is directly linked to an equal number of subscribers less for World of Warcraft. A lot of people that will play SWTOR end of this year are not currently subscribed to any MMORPG.