That was my whole point. If Bill Gates tomorrow thought his mission in life was to destroy Bitcoin by a sustained 51% attack he could and there is not much anyone could do about it. It would cost him a small fortune and his attack would be a massive sunk cost that he could never hope to recover but he could do it. Worrying about a pool trying to perform a 51% attack using the resources of miners by proxy in a very public and easily disrupted manner is just silly.
Of course my opinion is that any such disruption attack against Bitcoin would be a fools errand. Killing Bitcoin isn't going to stop cryptocurrency any more than killing Napster stopped file sharing. The concept is out there, it is open source, and uses strong cryptography. If someone spends tens or hundreds of millions of dollars to "kill" Bitcoin something else will take its place, and maybe they disrupt that too at even higher cost, and then a dozen "4th generation" crypto currencies will replace that and then a hundred or more replace that. Billions and billions of wasted dollars later you will probably see the "bittorrent" of crypto currencies. Necessity is the mother of all invention. It can't be stopped. Of course anyone smart enough to have those kinds of resources should be smart enough to see the futility of that type of direct attack (or be able to hire someone capable of that).
What is the point of killing bitcoin at huge cost only to have something better replace it. Napster was regulatable, but once it was killed it spawned not a clone but an evolution of competing ideas which became ever more resistant to oversight and regulation. Sure some of the ideas failed, some were poorly thought out, but people built upon those ideas. Future iterations of the same general concept hardened themselves against the weaknesses of the earlier versions. Today unauthorized file sharing is more widespread than it ever was when Napster was around.