In my opinion it depends on how the next two games (Arena and Peria chronicles) Release and the year they release. So far looks like they will keep vindictus running alongside Arena. I feel like Peria is aimed at taking away mabinogi players and attracting new players. Part of me just feels like they are pumping out content to cash out before that happens.
If I had to guess, depending on how peria chronicles is and how it peforms, it may start to pull players and then after that I'd give mabi anywhere from 2-5. It really depends on how nexons next lineup of games are. Either way mabis got a while before it dies. They are able to pump out content (lame content) but they are still putting time into the game and that = game still being pretty alive and kicking.