A number of different things could happen...
1. It dies from disinterest, and only a select few are left. They move on to a more lively forum
2. The site crashes, everyone regroups and have to create a new GangofFive. A large number of users never come back until 3 years later
3. The users keep united due their relationship with the forum and it's users/out of habit.
4. Everyone leaves GangofFive for real live, and tell their grandchildren about a time when animation actually meant something.
5. Everyone uses GangofFive until they die, and they pass on the LBT legacy through their children, who will later join the GoF themselves.
6. Production for LBT continues, and GoF continues.
6. Zombies invade GoF.
These are just based off my experiences with forums in the past (Union of destruction, dancing lawn, madness combat.net, etc.). When you have been to hell and back with a certain group of people, it's very hard to leave them.