Recently a disicission titled "Continued Growth of Gaming" take place in MBA Media and Entertainment Conference by a five-person panel which made up of executives from leading game and media companies.They sit around and talking about how to make a successful game based on a media franchise (like star wars), how to cut down development costs and so on. At the end they come to the MMOGs realm and make the example of WOW...
Audience member: How about MMOGs like World of Warcraft?
Frederic: I think there�s room for one, and that spot�s taken. Same with casual games: there�s room for one � Bejewelled � and that�s taken. I can�t build a spreadsheet and executive summary that can only mention one game as example of major success. And if WoW was really culturally relevant, there would be 30 or 40 million people playing it, not a few million.
Joesph: There�s another model for casual MMOGs that you see alot in Asia. Revenues are driven by micropayments for items, clothes, and that sort of thing. I�m not sure that will work outside of Asia, though.
Chris: MMOs, by their nature, cater to a hardcore market. The audiences they attract practically live in these worlds! Games like the Sims Online have been unable to attract a mass audience. It�s a challenge.
Joseph: There�s so much over-investment in MMOs. They can�t possibly all attract subscribers.
You can take a detailed look at this link