Regardless of the fact that no government would allow for fiat currency to be disbanded in favour of bitcoin, bitcoin also has a significant disadvantage in that it requires network infrastructure which physical cash does not. In the event of prolonged blackouts (such as those caused by cyclones or floods), there is already an issue with stores not being able to accept credit card payments - an issue that would also apply to bitcoins assuming that internet connections were down (or if your digital device with your wallet ran out of battery!). Because of this, I think Bitcoin will remain an alternative payment method as opposed to replacing fiat currency.