If you look into the history of modern MMOs, you will notice that most games in the genre are still up and running. In over ten years only a handful have failed and been taken offline by their publishers. That's not to say every MMO has had outstanding success. It's just that the minimum cost to keep these game worlds running is actually quite small. As long as current subscriptions can maintain the servers and minimal support staff, there is no real reason to take a MMO offline. In fact, most game publishers realize that shuttering a MMO is a little bit of a public relations nightmare. It's much better to keep a game running as long as there are any players willing to pay for it.