Yes, this is why we should not waste our attention on PoS at the moment, but there is an argument to be made for a PoW/PoS hybrid temporarily preventing a 51% attack if the economic majority is against the miners.
Also perhaps security is less of an issue for PoS if it could be structured as a supplementary layer which doesn't actually secure transactions, only serves to reduce centralization?
Perhaps , but a lot more testing and thought needs to go into this and we don't have time, so as a matter of practical urgency we need to start a testnet with us mining on it with as simple of a Pow change as possible... Keccak is the best candidate.