The world can not use single Currency,as every nation have different economic policy,that is relative to there natural resources and business activity.
Yeah you're right every country has its own economy so it's not possible to have only 1 currency for all countries. if there is only 1 currency then there will be a collapse of the country's economy because of the loss of a country's sovereignty to regulate its monetary conditions independently and certainly there will be a country that is profitable because it uses their country's currency. I think 1 currency will not work