The same is true for PoW, though. You can never guarantee that a single entity or a group of colluding entities will not gain control of most of 50+% of the compute power required. If the compute hardware is useful for other purposes than mining your particular coin, the risk is in fact greater - someone could build or buy up this compute power, destroy the currency, and then use the assets for other purposes, recouping some of their investment. With PoS, at least this much is not possible - anyone who would want to destroy the currency would lose their whole investment.