Just think about this, there are about 7.7 Billion people around the world. The last person who registered in this forum has an id number of 2,775,096. Let us say that all that 2,775,096 are all legit, no alts that means there are 7,697,224,904 people walking on earth that are not aware of Bitcointalk forum and they we're able to live normal on their everyday lives. So going back to your question of "What if BitcoinTalk forum dissapears", life goes on and perhaps someone from that 2,775,096 will stand up and create a forum just like this to continue what BitcoinTalk forum has started.