I think there is a common conflation of distributed and decentralized concepts; Many (most?) distributed systems require a consensus mechanism to operate (as they require coordination/state), but many decentralized(most?) systems don't, as each node operates independently and without a transitive state. As such, I have some difficulty in seeing PoS/PoW as "decentralized" (in a decentralized system, multiple different "end states" may coexist without being a problem), but rather more of a reputation model as a way to solve BFT. And BFT consensus mechanisms are - in my opinion - only one of several possible mechanisms, because (again, imo) "blockchain" and "public blockchain" make different assumptions on the underlying implementation.

Is PoS actually decentralized?

It depends on which PoS algorithm you look at, but some of them are generally considered to be as "decentralized" as PoW in terms of the security properties they give to their network under adversarial conditions.

Is PoW actually decentralized? It's more like multi-witness because miners aren't able to see the block they're working on and validate it before submitting back to the pool operator and they can only trust the pool to keep paying out rewards through non-blockchain means.

In ideal worlds as designed for PoW and PoS everyone has roughly an equal stake and equal incentives. Reality differs.