i do not think that it is going to happen since countries can not do without fiat currency there will be a big mess and i do not think that someone will like and support this idea.
It does't like that these countries can't do without their fiat currency but these days, they can just support Bitcoin and other crypto currencies what more is that it making it their currencies. But I do agree to what you just said after that, none on the government will just accept this idea since cryptos are just decentralized nowadays and it may be manipulated by whales.