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.