Bitcoin will never replace fiat cash. A coin everyone mints at the same rate will. It will also replace bitcoin and all of the shitcoins.
Mark my words.
Bitcoin runs off the fumes of fiat.
Bitcoin is a crypto-coin and its is a eclectronice cash coin also. Most of the people use it different type payments or trading.So bitcoin is not flat cash and it never will be flat cash because its price changeable and violent.