It will not replace, it will become the new concept of money earlier than 50 years. Fiat currencies allow the government to have a large amount of control of their economy. They can decide to print more or take some out of the economy through banks. A cryptocurrency would have to be linked to their fiat currency and be totally controllable, for a government to allow it in place of their fiat, but that would basically be a fiat anyways. So we would need new legislation or new governments.