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.
I know that bitcoin and fiat money have their own advantages in place, bitcoin is very powerful in the digital world and fiat money is very strong in the real world. with that difference, I am sure no one will shift to each other.