I agree with your general idea that PoW is still superior to, or at least more necessary than, PoS, but voting in a PoS system is not sybil vulnerable because y

I agree with your general idea that PoW is still superior to, or at least more necessary than, PoS, but voting in a PoS system is not sybil vulnerable because you have to lock a resource. If you assume an attacker has enough resource to lock through a sybil attack then they would have the resource available to control a PoW system anyway. The point is the resource has to be external (i.e. outside of the consensus system) in order to be invulnerable to sybil attacks. In PoW this external resource is computing power, while in PoS it is the resource of the system itself (which might be internal to the consensus system, but has to necessarily be gained externally).