Bitcoin can fall to 0 before 2018 for all we know. It's just highly unlikely. It's like how Bitcoin rising to 100,000 before 2018 is unlikely as well. Essentially, no one can predict what will happen. Bitcoin has seemingly defied odds that many have put on it. For now, however, we do know that we are in a bubble of sorts. There is a rise in btc price due to the upcoming fork. People will be dumping bitcoin gold very quickly. After that rise, there will be a decent correction period.