I don't think it can take over fiat system. However, they can used blockchain to improved fiat support especially in banking transactions. And I don't think that a country will used bitcoin as replacement to cash countries need to transaction to the universal currency which is dollar. So its gonna be hard for a nation to used bitcoin so not a chance that it will be like that in the future. What other nations should do is to support bitcoin, so together with fiat it will be a good balance, in my opinion.