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.
In long term though it can be possible.
Cryptocurrency legalization is a matter of time, you can already see the difference compared to early 2020