I think there's a difference between "blockchain + PoW" and "blockchain". Blockchain taken literally is just verification of history via hashing. It's a form of provenance that is hard to fake. Even if you are a central authority as to what transactions are being made, you can't rewrite history beyond any point that you've made publicly visible, without losing the trust of people who rely on you as a central authority, as to do so you would need to break the hashing algorithm.