I do not believe fiat can ever be replaced as the government exist.
The government could ban bitcoin so the value of bitcoin could drop to non existent to one's country
How can any government ban bitcoin,if they can they could ban any exchanges running from any particular country other than that it is impossible for any government to cap these things,but another hurdle is that you need fiat currency to invest in bitcoin,so it is a complicated issue and people will find ways to invest in it and fiat and bitcoin will co exists.