You cannot mitigate a 51% attack, if there's a very wealthy person with the desire to spend his capital this way.  It just has reached a level that even billionaires wouldn't dare to touch it.  Only someone who can create money out of thin air.  Only if you could print money, you could make this work, but you'd be found in one way or another.  The main obstacle that I see, as far as I try to understand their mentality, is that this operation requires them (or some scapegoat-entity to put responsibility to) to essentially "yell" to everyone who they are and what they're doing.

Even with all the money in the world, you can't kill bitcoin silently, as you would murder a US president, for instance.