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.
I totally agree with you but if am gonna add something it is that the main reason why bitcoin is not yet accepted as the global currency is because it has not been accepted by major government of the world say for example now the government of major countries accept bitcoin at that instance it will have great value that will make a currency of interest.