I don't think that will happen, even now I heard that the government already throwing accusations about bitcoin so I doubt that it will replace fiat. If they work on replacing fiat to bitcoin I know for sure that it will be hard to do.
it is true that most countries for now bitcoin is still illegal, because it can not be controlled by the government, but we do not know how to the future, because the development of the world of crypto is getting better and attract many investors to join. I can not judge it, but let's see together how it develops for the future