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.