Its an interesting question - but I think that any government that did this would basically be saying in no uncertain terms that they are the final arbiters of all money created.
It would mean that they are saying that you are not allowed to trade freely and that you are stuck with the currency of the country that you trade with.
If all governments said this then it would mean that they have created a monetary alliance that is stronger than Bitcoin because they could easily "outlaw" Bitcoin (using the legal system - they wouldn't be able to shut it down technically) and make their new currency legal tender.
If that were to happen then there would be no need for Bitcoin because it would mean that there would now be a borderless currency (which is one of the main value propositions of Bitcoin) and most likely the price of Bitcoin would drop.
If, however, they went about outlawing Bitcoin without providing an alternative - its likely that it wouldn't be a universal deal - there would still be countries that value the economic freedom of the individual above outlawing Bitcoin and would most likely not join the alliance - meaning Bitcoin could be mined and distributed legally there.
Maybe one day our leaders will understand that money is just a technology to help drive the economy and satisfy human needs and wants. Until then - buy Bitcoin.