FIAT money is our daily currency which has been existing here for hundreds of years, when crypto currencies like Bitcoin is squeezing their way up to become a world currency and sort. Can they co-exist? If so then what benefits we can get from them, altogether? Not separately.
It is possible for bitcoin and FIAT side by side because nowadays everything has been digitalized so that FIAT in the future will probably be digital and people don't need to carry a wallet to store FIAT currency.