There isn't a single supercomputer that could 51% the bitcoin network on its own. Traditional and even newer GPGPU supercomputers are actually a really shitty way to attempt a 51% attack.
And? You don't even need a supercomputer for that. Just buy enough GPU (or ASIC once they are released) and ta-dah, 51% attack. Cost? You can do it today with less than 20 millions (probably with 10 millions, and i'm speaking about buying all the hardware, buying the building and the infrastructure to put that into, hire enough ppl and blablabla). 20 millions $ is CHEAP if you are a rich person/bank/government. Very cheap.
What are you suggesting? That a political organisation might turn down a massive bribe? Maybe you're forgetting just how massive it is. We're talking about 51% of all transaction fees, plus 51% of all newly-created money, of a worldwide currency. That's a central banker's dream (well, they dream of complete control, but I think they're all aware that's never going to happen with a world currency). You really think anyone would rather try to destroy us (with no guarantee of success, I might add) than take a 51% cut? I don't.
"massive"? Some millions $ is "massive" for you? Oh LOL. In case you don't know the guys i'm speaking about (banks, governments, etc) daily spend billions of $. Dozens of thousands of millions. The 51% cut is peanuts for them.
Also remember, if bitcoin become successfull they will lose billions.
the current cost of a 51% attack is pretty expensive
I find it cheap. 20 millions? A soccer player gains more money in a year... clubs spend dozens of millions to buy one. Some years ago they bought Ronaldo for 90 millions.