Btcs volatility is the main problem. In a deflationary state, where the value of Btc rises, people will tend not to spend their coins as they would get more a day/week/year later. That transition from asset to currency might be too difficult for mass adoption.
Time will come bitcoin will be stable if people are using it and government will regulate it, for now, let's enjoy the fun of making money out from fluctuation.
Ofcourse bitcoin can't replace fiat because the main reason is that everyone here is only for fiat currency . And i also thinks that only one way to use our money fully , the money should be in fiat .
It can't because it solely relies its value.