I don't think so it will make the real money or fiat disappear. Government has the control to cash and they can reproduce it any time they want. But bitcoin, it is decentralized and there's no way for the government to re-product and it has a supply limit of 21 million so when it reached to that supply, there will be no way for it to supply and produce again.
i bet too that it will not happen ,disappearing ones country fiat is a lost for a country,government will not let it happen.
Yes that's it fiat is for government. And they will not allow this to happen as many people has been familiar of using real money. Not all people can also catch up the idea and usage of bitcoin as not all of us are computer and technology literate. Crypto currency will be used as an alternative method but will never make real money gone.