I think it is possible. The fiat may be replaced by Stablecoin. In this case the fiat will be controlled by the government
I don't think that crypto will replace fiat because yes, however, bitcoin is bought with fiat apabilla doesn't have mining tools
and it is very dangerous if crypto replaces fiat in the world currency can make a big loss if the country cannot regulate inflation that will occur due to crypto unstable itself
I think it will replace fiat because we know now the use of fiat is almost equal to nothing but as time passing crypto currency is taking value in the market, I near future I am sure only crypto currency will be there and it will be the single crypto currency we can use for buying goods and to make our life better after storing for long term as an investment.