What would be the reasons for the USD to fail? The only one I could think of is that the Union would disintegrate, Balkanizing the country, but why would that happen? You know the Euro since it's beginnings was given only a year before it would collapse and since then every year someone says this is the year the Euro fails and yet it's still working.
There could be a worldwide panic that causes the people within the United States and other Countries to exchanges all their money into a specific foreign currency that will allow them to hold close to the same value amount that they've started with. Bitcoin can begin that type of disaster, it would just require some time for it to happen.