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.
Until now my goverment still not admit bitcoin as legal payment, not illegal either since there's no constitusion or policy to restrict for using it. So at the moment it is impossible things that bitcoin can replace fiat (for my country).
But for future, I think it may possible to being replace as long as the price keep stable and security risk can be handle as soon as the problem occur.