I think the world is never meant to be one, the way which the world is right now with different nations is the reason why the world is developing through the differences in currency, economy, and resources.
Currency cannot be unified because it lies within the borders of each country's interests. Previously, the "Bretton Woods" meeting had suggested that the dollar should be linked to gold, the currency for the rest of the countries to be linked to the dollar, in which the United States plays a leading role and this leads to profits great benefit and it has been overused to increase the dominant interest of "money". This is one of the reasons, to unify currencies to a common standard, there is no need for any leader or middleman – yes, bitcoin can.