First of all, what we all care about in an online world is making a living world that stands the test of time, where players can keep having fun with their friends year after year. So success isn't based on how many copies the game sells up front. Success is based on whether people keep playing the game over time. And financially, success is based on whether people keep supporting the game by buying new content.
Second, I'll address this idea that the time of MMOs is closing. If the word "MMO" means the same game mechanics that companies have been hashing out again and again for the last decade, fine, let them rot. But if you change the word "MMO" to "online world", then I bet we can agree that "online worlds" are only going to keep growing in importance.