I think it's a matter of time that many governments will issue their own coins. They can easily launch a cryptocurrency for special projects ... such as infrastructure renewal, technology adoption, lotteries, etc. ... but it will not (in most cases) replace existing fiat currencies. The only reasons to replace existing currencies (that I can think of) are to eliminate debt (i.e., default) or the people don't believe in the new currency.

If it's to eliminate debt, it will be seen by the world as exactly that, and the results will be the same as a collapse of the country's currency and credit.

If people don't believe in the country's fiat currency, they won't believe in the country's fiat currency either. Both will be demonstrated to be backed by unreliable authorities.

If a government puts some thought into it, however, they can launch one or more cryptocurrencies for specific purposes and then use the potential profits (e.g., toll revenues for new roads or bridges) to help make good on the current fiat currency.

Do you think Governments are smart enough and cohesive enough to do it right?