Under no scenario is everyone in the whole world going to suddenly stop using bitcoins (this is the only way price can fall to zero). Its very easy to simply open up a new bitcoin wallet and transactions are very easy. Even if there was a very large government crackdown, the price could fall to about $100 but never below that. Also, some actual governments are investing in Bitcoins so its hard to imagine even that occurring.

Also, given the convenience and advantages of using bitcoins, it is highly unlikely everyone will suddenly lose interest in it. If mining becomes unprofitable, there are still a lot of people currently holding a lot of bitcoins.