PoS has been proven to be quite secure by now. The only reason Bitcoin doesn't change, is because miners would not agree since it would make their hundred of millions of dollars worth of equipment obsolete.
Security is not the goal of proof of work, and anyone who talks about it is missing the point.
Proof of work is a method for a network of independent nodes can arrive at a consensus without any privileged decision maker, a.k.a "distributed consensus."
Distributed consensus is impossible in proof of stake systems.