Just found this post which argues your point rather well. I didn't think of it in this way.
https://steemit.com/steemit/@cryptogee/steemit-com-is-not-the-decentralized-platform-you-think-it-is
So it seems you are right about the centralisation. As for manipulation/control, I suppose I can't argue with you there since the largest holders of Steem appear to be Steemit.inc and the biggest whales are able to manipulate and control what is seen or not seen. I really like this platform but I do hope it doesn't turn out to be a pile of crap.