There is already. Almost every country on earth has single state controlled currency. Fiat currency is already a natural monopoly on payments. Imagine if ever

There is already. Almost every country on earth has single state controlled currency. Fiat currency is already a natural monopoly on payments. Imagine if every time you wanted to pay for a train ride you had to put your money into an envelope, mail it to the United States, and wait for it to come back. That's VISA.