I believe in the words of Jack Dorse, the Twitter CEO, he said: “The world ultimately will have a single currency, the internet will have a single currency. I personally believe that it will be Bitcoin.” I can elaborate on the timeline, stating probably over 10 years, maybe even faster 
That will not happen in anyone's lifetime, trust me, no one will want a single currency to be used all over the world let alone a digital one. The reason why is because some countries will insist on the superiority of their currency over the other one, I mean look at US dollars, they're still contested to this day by China as a ruling currency for international context.Every country has its pride in they national currency, so it is very difficult to realize the whole world using only one currency. I also think it will never
happen in the whole world using only one currency, let alone make Bitcoin the only currency in the world, it seems increasingly impossible to happen.
Since not all countries accept Bitcoin, so how can Bitcoin be accepted as the only currency in the world. Moreover, big countries like America and China,
they consider their national currency the best. We have to be able to think realistically, for me Bitcoin in the future will only be a payment alternative
that is recognized by many countries, that is already a good achievement for Bitcoin.