1. No one believes in bitcoin (because of something).
2. World war.
3. When the power grid/internet has been shutdown.
Your option 2 and 3 may can destroy the whole world, not only the bitcoin.
Some computer scientists say quantum computers are moving closer to reality and will destroy the whole encryption systems which are being used now. Thus the bitcoin system is totally on risk and could be destroyed if the quantum computer comes into reality. If you google " quantum computer " you will get tons of result about how it effects on btc.