That is what the government don't want to do. They are not ready with the fluctuation of Bitcoin price. They can not control Bitcoin. I don't know for sure what they will do but they will not approve that. Bitcoin will not be used to replace fiat but work together with fiat will be good. So that is what Bitcoin will do in the future when Bitcoin grow bigger. But if my government use Bitcoin as currency, I will still use Bitcoin as the investment because I don't want to spend my Bitcoin to pay the transaction. I prefer to use fiat for the transaction payment and Bitcoin for the investment. Maybe that will what the government will choose so they can still control their country using fiat.
Any plans to take Bitcoin as a main currency should understand that volatility is a risk factor to keep in mind, no country will try taking Bitcoin as its main currency, for some reasons government has no control over Bitcoin, how do they regulate its value to meet the economy. Fiat has good importance, can't be against Fiat because it plays a crucial role in a country, making everyday transactions simpler. I agree with you when you say, Bitcoin should go in as an investment, not an actual currency (can serve alternatively), and adding to its value can be used for some large payments.