But as to the topic...of course 'they' could buy up all the bitcoin with unlimited fiat that they print at will. This is the U.S. gov't I assume you're talking about, but it could very well be any gov't. Why would they, tho? Who would make that decision? The U.S. gov't is at least nominally accountable to the voters, and those voters would have a hell of a hard time understanding why their elected leaders used taxpayer money to buy up all the bitcoin in existence. Even tho it wouldn't amount to a huge amount of money in the grand scheme of things, there would be an outcry that taxpayer money was squandered on something so unnecessary. Especially if bitcoin dropped dramatically afterward.
Thus there is no threat whatsoever that this is going to happen. Even if any gov't thinks bitcoin is a threat to their economy, none of them are going to go on a bitcoin buying spree anytime soon.