I think it is very unlikely that we will see a worldwide crypto currency issued by a central bank since that will require a government so powerful that it can impose his will above the will of the other countries and while the U.S is very powerful is not that powerful.
Never say never. There was no single country which was very powerful in the Euro zone. Yet, we did see the Euro replace the lira, drachma and the deutsche mark.
Eurozone is a different story - it is a monetary union. And it was introduced despite rules of an OCA (Optimum currency area theory) says it would be a foolish thing to do.
Euro (the currency) is not even 20 years old and there is high possibility (at least, I believe so) that European Union will split during next 20 years.
We also have examples of other currency unions existing:
US Dollar is currency of Ecuador, Panama, El Salvador, Puerto Rico, Palau and couple other countries.