I think that it is hard to happen. As we know USD is centralised currency. Government is very intelligent about maintaining the money equation throughout the country. The currency USD is constantly produced by government as per the needs of nation meaning, if by any means say example of bank, when it sense that their hard cash is exhausting it will be refilled by regulatory body immediately thus maintaining balance of USD in the bank. What this taught us? Well due to 'maintenance equation' of government it will never run out USD and due more and more production of USD they are actually adding more value to harf currency indirectly.

On the other hand our beloved Bitcoin is limited in number for now. By today we have capped limit of 21 million bitcoins in the world. Yes it is getting expensive but will be limited to certain amount one day. So at that time USD will surpass its value.

However, for instance if we think that USD crashes then there is big question that 'Bitcoin is decentralised'. So how will you distribute it in the world. What if one nation holds more bitcoin (more power) and others don't? This will lead to consequences. (Just in case if our USD drops!)

:-)