There are global currencies. USD is basically a global currency. SDRs are a global currency that logistics companies all use. Crypto will never be a global currency. It's a "gold standard" currency and too volatile. There are tokenised forms that could act as fiat, but government's need to control the printing of currency. Randos making token fiat will not manage them as well as governments. I know, I know, all government's bad, but, in reality, most are better than people like Bankman-Fried.