If large Govts bought all available bitcoins in an effort to wreck BTC it would be a drop in the bucket for them. ( and the Fed would only need to print the $ to do it) They would also be in a position to manipulate the market.

Furthermore if they embarked on mining using their massive resources it would probably push ALL small miners out of business.

Therefore BTC can not be regarded as a serious threat to their currency monopoly, merely an interesting diversion for the powers that rule the markets