fiat and USD is never die, bitcoin and fiat currency is a very different, we can see that fiat currency is have the physical form, and bitcoin and other cryptocurrency is not have it, so i think bitcoin and fiat is different things, so fiat will never changed with bitcoin.
Government won't allow to put away fiat though, it is the currency which they control. Fiat has big role for us to buy crypto. It can be hard to have crypto without fiat specially if you do not have any idea how to work another way to earn it. Fiat will stand still and used by people, it is more easy to earn and can by small amount of commodities.