This is just not true for a proof of stake blockchain. The higher the price of Ether is, the harder it gets to loan enough ether to stake to attack the network. And attacking the network or misbehaving does cost you a lot more dollar money.
On the other hand, the higher the price of Ether is, the higher the incentive to do so. The return on investment of an attack is independent of the price of Ether (assuming one can actually gain something from attacking a POS chain).