What is an MMO?
MASSIVE - Supporting Hundreds of players.
MULTI-PLAYER - must allow multiple people to play simultaneously in a persistent world
ONLINE - must be online to play
What is success?
Well no one will give the same answer. The truth is success is a measurement of happiness and money. For example, if I owned my own business and barely made any money but I loved what I do - I would consider that a success. But say I had creditors ...then I owe money and I am also quite unhappy. What if I make enough to pay my creditor but cannot pay my employees what they deserve and they are unhappy.
Now look at the gaming industry MMOs in particular.
The most successful game in the history of gaming World of Warcraft. Even if you haven't played it I bet someone you know close to you has. WoW became the standard of what gaming companies thought they should be. So now every single game that comes out says well its like WoW but the setting is different or Wow doesn't include this we do.
When someone steps outside of what makes WoW successful they either fail, or are looked down on like a failure.
EQ2 has been running as long as WoW ...it has a deep lore, made players happy with wants and fulfilled many roles. Yet no where near the player base of WoW, gone free to play, yet no advertising.
Warhammer Online - huge opening day, too big in fact they underestimated the need for servers and had long queues. They resorted in opening new servers and after the newness wore off they had to shutdown and combine servers. While they had lots of testing they failed by not including a 3rd realm for combat and could not support huge battles when 300 people entered a combat zone the servers just could not handle the load.
SWTOR - after a huge bump and push to take over the top slot ...failed as of this writing it looks like they are combining ALL servers just to get a reign on population. Froogle
Guild Wars 2 - nothing truly original and yet, so far, highly successful. 2 million subscribers when it opened.
There is no magical formula. WoWs numbers are slipping even though they have a huge expansion coming out. Almost every game on the horizon is free to play. How much time are you willing to invest ... and how does that equate to money and success for the company making the game? Can they paid developers with that success? Do companies just keep making games as fast as possible and move to the next one until they make a successful self running game and then come back to it and "fix" it? At what point is it considered a success?