Bitcoin couldnt replace fiat even it would be the most popular crypto on the future but as you said it would be a secondary option and it wont happen than it would replace fiat since people all over the world would still choose fiat rather than bitcoin on everyday living because they already use it for how many years as early they have the knowledge on how to use money and they only know paper currency.
You are very wrong. You think paper money was always used? Before the money, it was gold (bank notes), before the gold it was hide, meat, rice, beans etc... as currency. Paper money is very new. And we are already in the plastic Era which is the Credit cards.