PoS can not be decentralized and can not be secure. You will never know if an opponent has taken control of all or part of the nodes, all nodes can be compromised or hacked.
There will be no greater security than just the PoW, there will actually be less.
I believe its easier to hack 2 or 3 pools in order to control a network over PoW (and often 1 pool has more than 50% of it, so a single target to hit) than hacking all the nodes of a network. Pools also run more code than a node so attack surface is greater.