Single global currency can exist only in the global state situation, other than that the countries are too different to drop their currency in favor of the new common one. Euro is only the singular case, and still not all countries in the EU accepted it.
This is regarding official global currency. Meanwhile, there is room for 'unofficial' one - currency that is accepted everywhere without any issues, but not officially confirmed as global currency. In this case, Crypto - not necessarily Bitcoin - is the ideal option.