It's not just about driving out free players for server space issues. That is one part of it, but there is also the so-called "balance" issue. As Dipstik mentioned, ideally the oldest players, who are often some of the strongest players, leave after feeling like they've won. The fact that those people were (are?) selling their accounts was probably something Gree didn't plan for, newer players had a harder time following the game path that Gree tries to subtly coax people along, then stat explosion happened and here we are. Things would probably be very different if the original revolving door idea had worked out the way it was likely intended.