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.