Actually I don't think that cryptocurrencies will replace fiat money in the nearest future. There are still too many people who don't believe in crypto and don't know how to use them. What is more, government can't control crypto and that's why they don't really want it to develop as this way they will lose control over people finances.
In the same way I think, because I start from the main premise which is:
1.-Fiat Money = Money, Bitcoin = Money
2.-Bitcoin needs Fiat money to be able to give it a value and under which, it can be negotiated.
3.-Fiat money is needed for many people to understand the value of BTC and thus understand its price.
All these premises are necessary to be able to establish that there is no sense in choosing which is better, both are needed, that each one has advantages that people must take advantage of according to their convenience.