Open membership means that the chain stays alive as long as anyone in the world wants it to. This isn't true for PoS chains -- you must to acquire tokens to keep the chain alive.
> As an example, any sufficiently powerful entity can temporarily and affordably commandeer computational resources with the intention of disrupting the chain.
A sufficiently powerful entity can DoS enough staked nodes that quorum can't be reached, and thereby force a PoS chain offline indefinitely for far less energy. If they're clever, they'll buy some PoS coins first, so that once the offline nodes all get slashed, they'll be the majority staker.
> As an example, any sufficiently powerful entity can temporarily and affordably commandeer computational resources with the intention of disrupting the chain.
A sufficiently powerful entity can DoS enough staked nodes that quorum can't be reached, and thereby force a PoS chain offline indefinitely for far less energy. If they're clever, they'll buy some PoS coins first, so that once the offline nodes all get slashed, they'll be the majority staker.