See if you can follow this reasoning: WoW has ten million players, which is nice and all, but there are actually 800 million teens in the world. Therefore, since Blizzard hasn't reached even 10% of them (80 million), WoW is not actually a success. That's what Craig Sherman of Gaia Online (a casual, browser-based MMO) said to folks at the M16 Marketing conference in San Francisco this week. He claims that WoW's subscription fee has hampered its growth, and that it would be even bigger if there was a free-to-play model.
But his reasoning is unstable there to say the least. Part of the reason WoW is so successful is that Blizzard has had the cash to put up for new servers, new content, and a brand new HQ, and with a free-to-play model, they wouldn't be making nearly as much money as they are. Not to mention the quality of the players -- in my experience, part of the reason WoW is such a good game is that when people pay to play it, you often get a much more interested and involved player base. And of course, while yes, WoW hasn't reached a larger fraction of its "potential" player base (however you define that -- what makes Sherman think that Blizzard is targeting teens at all?), anyone who thinks a 10 million player MMO is "not a success" needs to examine the rest of the MMO market more closely.
Will there be a game bigger than World of Warcraft? It sure seems like it -- at some point in the future, there should be a game that does go free to play and does hit on all the marks -- casual, hardcore, serious, fun -- that World of Warcraft does (in fact, maybe WoW itself will someday open up a free-to-play model). But to claim that WoW has somehow suffered from its subscription model is pretty far from the truth.