Can't answer for the mod but since I'm a part of the community and have spent nearly or over 4years actively participating in discussion here, i can say the dedication that the community members have for the forum is what's keeping it alive, not the signature or bounty as others might say. Take those features out and the forum would still be functioning although there's no doubt it won't be as active as it's now but it'll still be functioning.
The mottos has always been 'Don't feed the trolls' and that have helped kept the conversation clean to some extend, some argument here and there but overall the community has always found a way to go pass the trolls that pop up every now and then. The moderators are also getting some kind of compensation for their service but overall their service is seen as a volunteering service to the community so I guess that's what is keeping them going.
We have active members here, doing something to help in the continuity of the forum either via their selfless service to the forum like spam, scam busting or quality contributions to the ongoing discussion on the forum.