in my opinion all possibilities can happen that fiat currency is replaced with crypto currency but requires a process and a long enough time to convince all crypto currencies to be worthy of fiat currency
For Bitcoin I think it is possible and it is going to happen very sooner. This year there was a hurdle for the promoters and other users of crypto because of the restrictions and the market situation but sooner it will be good and again the crypto will come on the field and will take the place of fiat in the remaining fields where it is still not implemented.