There is a much easier way for the government to take over Bitcoin than 51% attack and selfish mining. Simply tax the coin and per footnote [2] all coin is tainted regardless if you used Tor or not, so in effect regulate every pool and exchange. Regulation can accomplish much of the same goals as takeover. Then later the large corporations beholden to the government buyout the pools and exchanges, and then the drift towards cartels and corporate-fascism.