Cryptocurrency can never replace fiat currency. Fiat currencies are more or less like a generally used currency used by many countries in the world.
If fiat conditions are not possible to maintain balance in the majority and many countries experience bankruptcy, then they will form an MoU to use crypto currencies. For now it has not been seen, but the wave of inflation will come and only developed countries will survive, so alternative currencies have been considered from now on without any binding restrictions. Controversy does not mean not considered.