I think this can only be possible when all countries uniformly has developed to such level that electricity and internet is available all the time and is very very cheap. Smartphones will be in hands on every person and every person be educated enough to know and use the technology.
Replacing fiat money with cryptocurrency is a utopia. This certainly will not happen, because fiat money is used on the black market (they work in conjunction with the government), it is difficult to track the catch when you use cash. In any case, the government will not want to switch to an economic system that is decentralized. After all, we all understand that we have a centralized economy at a given time.