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
No one would accept that, government wants to rule its own society and it is really impossible to have one goals, politics and greedy people will always look for their benefits and it will affect many things including currency and economics, as per now, having one global currency is just a dream and still
far from reality.
Of course, there are a lot of economic dramas in the world to make every economy in their country improve, look at how the USA government is carrying out trade wars with Turkey and China, it is a form of government policy that wants to make their local currencies have a high exchange rate, so it's not gonna happen the world will only have 1 currency