You should be able to keep a AAA MMO around for at least 2 years, as each expansion pack is 1 year of developement time roughly. Most MMOs last only 2-3 years, so it is realistic. Only the most popular MMOs last 5-10 years. I think the longest I've had an MMO was almost 10 years, maybe 6-7 I forget. But that was because I was i the post game and could take the maintenance cost hit because I had over $1 billion. And expansion packs honestly should drastically increase server costs not reduce them. As more people buy a game, more servers are needed to hold more people. I think it should be that server cost go up, and go up really high the week of an expansion pack release, but go down as the game wanes in popularity.