"it should be trivial for the NSA to take over 51%+ of the network"
Should it? Where would they take that kind of computing power from? 700 petaflops? The top supercomputer in the world does 20 petaflops, for comparsion.
Not sure why you think it would be that hard with customized hardware, but let me ask the question a different way instead.
In a post-CPU, post-GPU, ASIC dominated world, what rational bitminers (other than botnets) would compete against the NSA, or any malicious attacker, who was willing to mine unprofitably? Say even below the cost of electricity, with hardware and bandwidth costs of zero.
If NSA use custom ASICs to mount a 51% attack they are injecting bad data into the blockchain, which is destroying the value of bitcoin.
But even if they don't do this, and they become part of the economy - it's easy for them to create havoc by flooding the market and devaluing bitcoin, then buying all the cheap bitcoin, then flooding the market again. Do that a few times and they'll have removed anyone but gamblers from the bitcoin economy.
Should it? Where would they take that kind of computing power from? 700 petaflops? The top supercomputer in the world does 20 petaflops, for comparsion.