Ever heard the phrase beating a dead horse? At point, and what kind of catastrophic failures will you have had to endure, before you'd consider closing your forum? I tend to stick to specific game forums which have the potential to die out if/when the game dies out. At that point I would probably have to either close the forum or move the focus to another related/similar game. If your niche is different and does not have a natural expiry, would you ever consider closing or just keep on going forever? Does a forum have an expiry date?