The whole thing about a government buying all the bitcoins is that point blank is it wouldnt happen. If they somehow found a way to make bitcoins useless... then the entire bitcoin network would just move to a different type like litecoin. The process would basically continue and keep continueing for quite a while.
And, I know I shouldn't say that:
There wouldn't be anything more awesome than that. The huge buy orders coming in everywhere would raise prices. Someone buying all bitcoin will be in a total pain. It would be the ultimate pump and dump EVER.
Some people are gonna get really, really rich, when the government does something like that.
Granted, that last 10-20% to the selling party might loose a fortune, but everyone else will be licking their fingers, waiting for the gov to pull it off with the next currency.
And then, then you guys know, how being a big bank boss must feel being bailed out.