My opinion here is if goverment does issue a fiat cryto and force people to use it. It would only affect citizen of that country and it would be less popular than Bitcoin. Value of a US govt backed cryto would be tied to the US dollar. So it would just be a FED back Tether (USDT). If you see USDT now. It does not grow much and used mostly as hedge when Bitcoins and the altcoins are down.

But I do see a plus if government does adopt a cryto. I think adoption of one cryto would make other crytos popular because we can easily covert 1 to another.