For me, i do not think bitcoin can replace fiat totally, because when their are larger transactions to be processed, it takes hours or even a day, except you have to pay Higher fees to process your transaction.
It never takes hours or a day for me. It used to be like that in 2017/18 when the network was spammed by small transactions to make it look like BCH is the better coin because it's faster.
Most BTC transactions these days are included in the first or second block, which means that you will have to try hard not to get a confirmation in 20 minutes.
If instant transactions are a must for your business you can use a payment processor like most online stores do.