I'm not sure that Bitcoin will replace fiat. I do not see any sense in it. Bitcoin has already taken its place in the economy and in the life of users. Why do we need to Bitcoin replaced the dollar
Bitcoin is supporting our digital financial but not for the our real financial. bitcoin is not taking the place of fiat. and bitcoin still have a lot of the home work with the government authorities about to the legal statue.
yes currently its look like impossible but i think it is possible but late in future not so early. when all people will accept bitcoin and will start bitcoin instead of fiat. then fiat will become useless and bitcoin will surely take its place.
Bitcoin could reach major adoption and yet it will never replace the dollar the US government is not going to allow for any currency other than their own to be legal tender in the US fortunately bitcoin does not have to replace the dollar to become the best currency in the world.