I heard this from this asian conspiracy theorist from many years ago like 2011, he said in the future there WILL be a one universal currency. Which he meant every where in the world will use that currency and other currencies will disappear. But this universal currency where will it come from? bitcoin or from the fed? I think in the future there will be 2 true currencies, that 1 is the bitcoin and the other will be from the fed. Only time will tell.
I do not think it is possible that we used only 1 or 2 currency for the whole cpuntry in this world because the economy of one state are reflecting in the value of currency. If we used 1 currency i think you can not identify which country has a good economy or which is the riches country.
But bitcoin would be listed as centralized currency if they can provide physical coin and paper wallet in the future to make available to used by all people.