I'm not sure if the government is afraid that bitcoin will take over fiat. You will still need to pay taxes in fiat, which will be a portion of every transaction - sale/income/expense. Fiat will have a place in the economy because it's what people trust, regardless of its fundamental flaws.
Yes, as long as you need to pay taxes in fiat, there will always be a need for fiat. But think of a country with a huge trade deficit. If Bitcoin becomes an established international currency, they will be happy to accept bitcoins which can then be used to bridge this deficit. If dollars are welcome in places like Argentina, bitcoin will also find a place.