It is understandable that Bitcoin can not be a global currency. At present, the US dollar can be considered as the global currency. It depends on the value of the currency, the legitimacy of the country, and the popularity of the currency. Bitcoin is valuable but not yet popular, and is not considered a legitimate currency of many countries around the world. But I believe in the future, with the development of the Cryptocurrency market, Bitcoin will become the "US dollar" of the Cryptocurrency market.