The first country to eliminate fiat and convert to crypto fully would be one which has a failed economic system such as Venezuela, where people are looking for and need help desperately, Bitcoin and crypto could be what they need. Just imagine if they turn to crypto and in a few decades everyone there became rich? It is definitely possible!
The problem with Venezuela is that it has a big number of people I think for bitcoin to be adopted by a nation the nation needs to be small a few hundred thousand people at most, and that has high standards of living, because bitcoin is not going to be as easy to understand to someone that has never had access to technology, however such a country probably does not need to adopt bitcoin in the first place.