Bitcoin is a common digital currency for bitcoin-accepting countries, as if the world accepts bitcoin, bitcoin is seen as a global digital currency for the whole world.
Of course if Bitcoin accepted as a common currency by many countries and if Bitcoin become a global currency the world business will be developed and improved for better but it doesn't mean that US dollar, Euro and other major currencies will have no place in the world but I think they will pay there major role like they do at the moment but during international transactions business individuals and others will have an alternative option to select Bitcoin over fiat due to its transaction speed and convenience and definitely if Bitcoin becomes popular as a major digital asset most people will do