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.
I think its happening now, we enjoy fiat money and cryptocurrency at the same time and if you see the result right now, we have so much options to transact around the world. Fiat money will stay, it will not die so both of this currency will co-exist and it will help us to live in a better way.
Cryptocurrency attracts many investors because of its technology, and if people continue to support this one soon we will witness a great financial system than can beat banking system, well this is purpose of a decentralized market at all.