A critique I have for your statement is that Steemit Inc. really does own enough STEEM to effectively "own" it in the sense of governance. If they don't want someone to be a witness or they do want someone to be a witness they have the voting influence to decide.

Another example that can be used is Steemit Inc.'s delegation to Steemcleaners. In a truly decentralized atmosphere, you would not have a "blacklist" of people that a massively powerful account could punish for behavior they dislike. If Steem were completely decentralized, including token distribution, it would mean that individuals could downvote you or put you on their own blacklists but not an organized collective such as Steemcleaners.