Perhaps the reason this wouldn't happen is because there's nothing in wrecking bitcoin for the super rich? 50million to do it maybe, but in destroying bitcoin's credibility, there'll be no value resulting from any dastardly deeds I'm pretty sure.
What if the obvious and financially strong enemies of Bitcoin (Banks, PayPal and likewise) do a 51% attack to save their own ass? They sure have got a hell to win if BTC loses credibility.
Would this not require some serious infiltration on the part of those entities? I'm really not sure how it could ever be orchestrated, let alone with the sheer number of nodes necessary to overpower the network before the others realise what's going on. And even if it does happen, that's some serious money laundering and probably wouldn't go unnoticed in court!