Like for example I want to buy a single cupcake on the street worth of 1 usd but need pay 1 usd more for the fee? Or receive my paycheck this afternoon with bitcoin and wake up the next day just to find out my money decrease from 5k usd to 4k cause market volatility? At this moment I think we still need fiat money. Until there some cryptocurrency that solves the problem about confirmation time, fee, and stable price.
IMO, if we want a stable one then why don't we use USDT? Though it doesn't backed by anything and so other cryptos neither bitcoin? So yeah why not if our concern is volatility? Personally, usdt is quite enough to be a currency and leave bitcoin as a pure asset.
Crypto replacing fiat will be a big leap in humanity as it will revolutionize the payment system yet this will be a long process so I'm just looking forward in crypto as an additional mode of payment like why fight over fiat if we can add cryptos, isn't it?