Even if the NSA or a hacker with a big botnet do mining, it wouldn't change something. They will have the 50BTC for creating block each time they create a block. Where is the problem ?
At last there will be no more BTC create  (when the limit of 21MBTC will be raised) so the only reason to mine is the tax. Which is low. So few people will mine, so the difficult will decrease so many people will mine, so...
It's the invisible hand.
Mining today is a temporary situation until all the BTC are created. The 50BTC for each block is just a way to create ex nihilo BTC. It's unfair, but how could it be ?