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
It will not replace traditional money, but there will be people, who will really prefer using the only cryptocurrency when it becomes more popular. I will understand them but will not support. Fiat is needed.