1.Crypto currency is more secure and faster transactions than fiat money via long distance and transactions like sending money around the world crypto is fast, safe and more secure then fiat money in this case fiat money is not good and slow in long distance transactions.
2.Fiat money is more faster and more easier in hand to hanp, face to face transactions and offline transactions in this setuation crypto currency is not good.
This 2 reasons why crypto currency and fiat money will be a powerfull combination in the future.
Also there are some case that fiat money can do and crypto currency cannot do, lets the example if you are in the plane up in the sky and you want to buy a cup of coffee or some drinks you can use enternet there so what are you going to do? In that case fiat money can solve it except crypto currency can solve the problem and can use in offline transaction in the future.