It is true that Bitcoin is growing bigger and right now is the best crypto coin in the world and a lot of people are also investing in Bitcoin however bitcoin can never replace Fiat currency reason being that every country has a flat currency and this their currency is what is being used to trade in the world market so it is impossible for Bitcoin to replace it, flat currency tells if your countries economy is growing well or not, if your economy is doing badly it will affect your currency in the world market, there's how the world Bank and the world trade uses this Fiat currency to balance things.
And again bitcoin is highly volatile in nature so basically bitcoin is for investment purpose one can use bitcoin to purchase items but using your Fiat currency to do that is better, and those that are investing in Bitcoin still need Fiat currency because they use it has their backup funds which are there emergency, reserve and float funds, you can't have this funds in Bitcoin or any other cryptocurrency because of the volatile nature of it, so it is impossible for Bitcoin to replace Fiat currency.