Bitcoin may seems impossible to repalce fiat at this moment but in future this is very much possible. Bitcoin adoption is going fast and by 10 years it is very much possible that bitcoin will replace the fiat.
When everyone will start using bitcoin, the fiat currecnies will fade away automatically.
I'm not sure the creation of bitcoin that satoshi did to compete and replace fiat, I'm pretty sure that's not the goal. Even in the next 10 years bitcoin will not be a competitor to fiat nor will it replace it regardless of how well adoption increases.
Bitcoin is a good payment network compared to fiat but cannot completely replace fiat as the government will not legalize it as a legal currency or tender. Bitcoin is a decentralized currency that allows users to get privacy and is the best alternative for those who like good financial privacy.