Last year Bitcoin was labeled as the worst investment of 2014, however, bitcoin believers can topple fiat currencies as the widely used mode of payment all over the world. The technology behind it and bitcoin's scarcity can both pose a threat and bitcoin's selling factor. But the question is, can bitcoin really substitute dollar or other currency sometime soon?
Yes it can replace fiat but it wont, its more likely we will see gov replaced by a new type of donation based gov and therefore fiat will just become worthless. This seems like way in the future, gov is very strong atm.