Yeah right government might choose their national currencies rather than bitcoin. Maybe if there was a new country and adopt bitcoin as a currency that could happen, but I think it is impossible. Fiat is not likely to be superseded by the bitcoin whenever I think. In the future will be developed into a bitcoin digital currency, not the fiat.
Bitcoin will be just an additional currency and the technology around will give difference in the usual fiat transfers.
Fiat will still dominates all money transactions in the world since it can support physical transactions unlike in bitcoin that only lives in digital world. But yes using bitcoin is really more less hassle when dealing on online transaction but not on physical transactions.