Why do people throw out the idea of the government attempting to buy up every bitcoin? It's insane. If the government did want to kill Bitcoin, do you think they would spend a billion dollars to do that? Or do you think they'ed spend $10 or $20 million, and have their own ASIC's deployed in order to over run the network? Or would they just locate each of the big mining pools, DOS them and/or block their traffic and leave Bitcoins difficulty so impossibly high that the rest of the network couldn't confirm blocks in any amount of reasonable time?
Buying out bitcoin would only cause people to rush and hop into the next coin, awating an even bigger buyout. Done properly, a decent attack MIGHT have the effect of making most participants wary of even trying to do this again.
Think about it.
They could even bump up the stakes by quietly having an ASIC farm in waiting (wouldn't have to be as big as in your scenario), DDoS all pools so difficulty plummets, then when difficulty changes, bring the farm online and double-spend as much as able to erode confidence and perhaps even convince some merchants to stop accepting BTC. At next difficulty change, halt the DDoS against pools and get everything in order to have all BTC exchanges served at once, while letting everyone mine until next difficulty change. At next difficulty change, shut down and seize funds from all exchanges able, then point their gov ASIC farm back to their old job of bruteforcing encrypted messages, letting the network semi-organically bring itself to a difficulty death spiral due both to the price plummet (due to many successful double-spends and sudden failure of many exchanges) and the huge dropoff in hashpower from the gov ASIC farm being pointed somewhere else.