There's one more argument that Steem is not as decentralized as we might wish and this might not be so obvious on the surface - not all Steem witnesses run their own nodes, even when in the top #20.

Outsourcing Steem node operation should be a red flag to anyone who is choosing their witness, and since there's not many people who can effectively run Steem nodes, those nodes end up being controlled and maintained by just a handful of individuals.