Cryptocurrency replacing fiat isn't that easy, to experience a complete usage of cryptocurrency same as the fiat it needs a large scale adoption. Compared to the previous year what we experienced by this year is a massive fall, but adoption has been taking place in a gradual manner. With this I believe, in future can experience cryptocurrency usage same as fiat but replacement won't happen.
I think some country should test this conversion from crypto to fiat. They should stop all fait transactions and ask people to deal only in crypto for some time. This will help us know the problems which people might face if there were only digital currency.