It could become worthless, just like the us dollar could collapse and could become worthless. But I don't think we'll see it happen to bitcoin.
Yes, both are unlikely to happen but theoretically can happen. Most important thing about BTC is that it has the value just because people give it the value, cut the demand and price will come tumbling down. While in case of USD the government gives it the value so unless the government is to fall usd won't fall.