It depends on the country. If a country is 1st world country just like the most states in Europe, they can replace there national currency with crypto because they are technologically advance while in the developing countries and the third world countries like in africa, how can they adopt cryptocurrency if in the first place they are not aware of that? so it really depends on what country.
the assumption that developing countries need preliminary policy from developed countries is true, but in fact developing countries are able to make cryptocurrency move more decentralized due to the factor of economic need is very large.
I think if one of their banks is able to cooperate with the government, then crypto power will emerge from the developing countries first and keep FIAT and crypto moving in the same blockchain system.