In a PoS currency how do you guarantee that a 51+% stake holder remains a benign actor?
It's only logical that the 51% group of holders will not have bad intentions against a currency that they themselves have a majority stake in.
For individuals behaving rationally in a closed system that is true. With entities that may have some overarching agenda, such as, hypothetically, the dominance of a competing currency, then the cost may be well worth while, despite "collateral damage", as it were.