it can only go to zero if a major flaw is found in the protocoll or the encryption is easily hackable some day in the future.
if nothing like this happens i would bet a lot that it never goes to zero again.
bitcoin is an open source project. everything can be solved in quite a quick fashion. a flaw can make the price go down a lot, but not to zero.
^ this but if there was some fatal flaw it might go near zero in a hurry. Since it didn't happen until now I don't expect it to happen (extremely unlikely - I see WW3 more likely than that), after all the bounty is the whole market cap.