FIAT will still be useful because the government prefer fiat and they are in charge of the currency. they determine the growth and they can easily trace any transactions and nail down fraudsters
The problem with the government is it still controlled by people, so we cannot expect that they are transparent unlike a decentralized system.
Bitcoin will become more popular because the real transparency can be seen when we are using the blockchain and no one can delete and undo the transaction.
We're almost at armsreach of that 10k target this year and fiat's not even flinching. That goes to show that fiat doesn't mind bitcoin and crypto. Whatever happens to.bitcoin, fiat will be fiat and unless majority of the people will have the means to transact digitally, its not going to lose value