Fiat currency and cryptocurrency are different. You only see the same function of the two. Compared with cryptocurrency, it is even more advantageous. It is only in a shorter period of time. But you ignore that the legal currency is managed and controlled by the government. Currency is not good. This alone determines that cryptocurrency cannot replace legal tender in the short term.
Even in the long term, I don't think cryptocurrencies will replace Fiat currency. Wouldn't we have said years ago that Gold should replace Fiat because at a point when Trade by Barter was still wisely done, gold was also a means of exchange and i still believe it is even today but not as it was back then. Crypto will exist for those that understand it and Fiat will exist and continue to be the Governments global means of exchange and economic control.