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.
Of course bitcoin and fiat can co-exist and both will have separate means in real world economy.
Lets say bitcoin can be used in businesses to transact globally without limitations and with fiat we will count and give value to bitcoin in market. Without fiat we couldn't give value to any coin/asset in our daily life