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.
CBDC's Central Bank Digital Currencies will replace cash meaning cashless societies. There will be no paper fiat cash soon.
CBDC's are like centralised altcoins that will have the official central bank/government mark of approval for them. They will just be centralised official stable coins.