I do not believe fiat can ever be replaced as the government exist.
The government could ban bitcoin so the value of bitcoin could drop to non existent to one's country
Government will always be there so there's no way for bitcoin to make the fiat worthless. But who doesn't like when the price of bitcoin starts to reach at $10k? No matter what will be the price of bitcoin, decentralize is decentralize and government will not advice people to depend on crypto currency, there will always be fiat.
I think it is all relative. When the bitcoin price break $10,000. It will soon be $100,000.