In my opinion, i don't think there would be one global currency it is because each countries has it's own government policies and laws.
You right, that's a simple but precise reason. it is impossible for the whole world to have only one currency whereas every country has
its own government and rules based on their ideology and the state of their society, unless the whole world has only 1 government, but ts not gonna happen
First of all, one day of money trading is worth trillions in Forex and having a single currency will end this trade. Besides as you said, the law of the governments. For this, the entire world must unite and become the only country. Human greed can be one of the reasons that cause it. Because, Bitcoin was the only one in the market and the feeling of earning created thousands of tokens in people. However, Bitcoin could be one.