You've got to remember that bitcoin is small-fry.
If it wasn't, it would be an absolutely trivial matter for a US governmental agency to use a 51% attack or something similar to bring down the network. They have some of the brightest minds in coding, and a eleven-figure budgets, we wouldn't stand a chance.
You give "them" too much credit. I know they look good right now, you know landing that SUV on the Martian surface and all. But the fact remains: the current hash rate of Bitcoin greatly exceeds the power of the fastest know supercomputer. Additionally, a 51% attack doesn't destroy Bitcoin outright. It only allows them to interfere with the generation of new blocks, eg, they can block transactions and monopolize the block reward. In *theory* many miners would drop once they hit a point that it became futile to mine (eg, a SUSTAINED 51%+ attack), but i got a feeling that people dont' like getting rolled and you'd see hash rates rise to whatever challenge threatens Bitcoin!