It will never replace fiat, why? First of all, Bitcoin is not designed for replacement fiat. It designed for fast transaction, and to secure our money. . However, fiat can modified but replacing bitcoin will not be effective.
I also do not think it's convenient for anyone to replace fiat with anything, there are many daily transactions that are much easier and convenient to make through the fiat,besides that because of the high volatility in the prices it is not possible to determine an immovable price for many of the goods or services that are provided and that these need to preserve their value and so only the fiat can handle in a more effective way.
Bitcoin cant replace fiat because of high transactions, long transaction time and because of its volatility. The high transactions cost of bitcoin will makes it hard to replace fiat. Its transactions cost is too expensive for small transactions. Unlike in fiat, youll just have to pay for the tax in every item you buy and its not that expensive unlike bitcoin.
The long transaction time in bitcoin can take at least one hour to fully confirm your transactions. So if youre buying stuffs in a store, youll have to wait for an hour to confirm it and its kinda annoying especially if youre in hurry.
Its volatility price could also prevent bitcoin to replace fiat. Its price keeps on fluctuating and theres no stable price for bitcoin. Theres no exact amount on the things that you wanted to buy. Like when you buy clothes in the mall for $10, and youll see its price in the morning that it only cost $5 or $12, it could really annoy you.
Lastly, majority of the people doesnt know anything about bitcoin. And its not being legalized and accepted in all countries and malls.