They didn't go bankrupt "because they tried to monopolise silver", they went bankrupt because COMEX changed the rules of the game.
You don't buy *all* the bitcoin, you would just buy enough of it.
You are the US govt. You have infinite money. You buy anything below market price, and put it back up for sale at some percentage higher. The market will trened higher, and you will gradually accumulate bitcoin all of which was bougth for less then the market price.
The more you own the more scarce a commodity it becomes and this is what will support the price. You do it throughout the inflationary period of bitcoin. At which point bitcoin has replaced dollar, you as the govt own most of it, all the while you have also been cornering the block chain by ramping up govt controlled cpu power. You then set your miners to ignore zero fee / low fee, thus effectively enforcing a minimum price for transactions that have any need to go through quickly. Now you are taxing the world. Welcome to The New New Order!
Why does anyone think that this is not already happening?
The defense is slow and continuous selling from the large holders and exchange for real assets such as gold/silver/property/asic miners/means of production.
This is already happening now, and more Bitcoin enter the market from the miners. It is a wise design with much safety built in.
It becomes very expensive to kill one of these crypto-currencies, and a new one can start.
Try putting the cat back in the bag.