It will take a very long time for Bitcoin to become the world wide 1 currency. There are still a lot of country that don't know or believe in Bitcoin.
Perhaps digital world wide currency is possible but if you change a Fiat currency with bitcoin to become a worldwide currency it is impossible. A lot of things to consider... Not all countries are already accepting bitcoin, they didn't even allow their people to use it. Bitcoin is too volatile to become a worldwide currency. Also it always needs internet connection to use and make transaction. Not all people know how to use it specially those who live in a remote areas.