Bitcoin won't replace fiat currency, it won't happen because fiat is decentralized which is used in the real-life governed by those authorities. Imagine if bitcoin would be the standard currency or replace fiat currency, a lot of small businesses and those unfortunate people without any knowledge about cryptocurrency would make their life so hard because of bitcoin
Firstly, I don't know if it was a typo, but fiat is not centralized. Also, small businesses and others who don't know about crypto doesn't really sound like a valid reason. The use of cryptocurrency as a currency is not complex. It's something that wouldn't take one's time to learn.