Someone to buy all Bitcoins, well that guy would need to be... well rich is not enough. But if there was a guy like that, after buying all Bitcoins he would not be rich anymore. But your first choice is a possible one, at list to some extent. Possibilities for something like that to happen are really small, almost impossible. But even if something like that is to happen, it would not happen over night. It would need a lot of time, and not to speak that there would be countries who would not give a damn about other countries decisions. It is almost impossible for Bitcoin to die, at least not in one day... or week.