I think the point you seem to be missing is: *he can't*
Looking beyond the irrational actor argument, for which you haven't provided a counter-argument, the NSA could decide that Bitcoin is a threat. They could have a team of hackers break into a mining pool that controls >50% of the hash rate and perform the following operation:
1) Transfer all of the pool's coins out to an exchange, sell them in small chunks for USD, and wire to an undisclosed location.
2) Initiate a double-spend attack on the coins that were sold, thus stealing them from the wallets of the general public, and causing Bitcoin to fail in the eyes of the public.
The Bitcoin Foundation thus has 2 options on how to respond. They can either:
1) Leave the new blockchain in place. Have an angry population that was stolen from. Bitcoin loses confidence and dies.
or
2) Revert the blockchain such that the double-spend never took place. Consumers keep the Bitcoin. The largest Bitcoin mining pool goes bankrupt. Miners do not receive their rewards and become disillusioned with Bitcoin. Hash rate suddenly drops substantially. Bitcoin becomes open to further >50% attacks due to low hash rate. NSA repeats the operation until Bitcoin is dead.
Or... the Bitcoin Foundation could be proactive and fix this before we have an attack in the first place.