Bitcoin is already replacing FIAT. FIAT is garbage and has terrible properties. Bitcoin is far superior in every aspect when it comes to durability, divisibility, portability, fungibility, and it being recognizable.
Bitcoin cannot actually replace FIAT. Yes, the use of FIAT currency may be stopped to some ectend due to bitcoin but bitcoin can never replace foat as fiat is a centralised currency under the acts and rules of government whereas, bitcoin is a decentralised currency with no restrictions.
I agree that Bitcoin can not replace fiat money because even the users of bitcoin are still using fiat money for their day to day tranactions. The people who are using fiat don't use bitcoin so replacing fiat as a bitcoin is beyond impossible. So for time being we should think about developing bitcoin so that everyone knows about it and digital money gets famous too. Maybe in future this can happen but for now we need to focus on our earnings.