Cryptocurrencies don't have such infrastructure and legal and economic basis to be used exclusively without fiat. It's not fully accepted, it's decentralized and mostly without regulation.
Those are among main reasons that we all stil depend on fiat money and cryptocurrencies are mainly an additional income.
So it's the best to combine both and use the advantages of crypto and fiat according to the opportunities. The ratio between cryptocurrencies and fiat will change with time but still I don't think that cryptocurrencies will prevale and become mainstream so combination will be necessary.
It will be the best of among all the best ideas because it has a big opportunities to the world and community to bring a better world abd economy as the years goes by ,the fiat and cryptocurrency which consider the key to have a good economy and better future in life, this ideas has many advantages such as government, legality, institutions, market and many more.