81% of the block production for ETH is controlled by 5 mining pools...that is not the picture of a blockchain that is "totally decentralized".

https://etherscan.io/stat/miner?range=7&blocktype=blocks

As far as the possibility of Steemit Inc to dictate which top witnesses get elected that is a possibility but so far the stake in the Hive account@steemit account (which has about 75% of their total stake) has not been used for that purpose.

For Steemit to dump their stake on the market they would first need to power down. That will take 3 months to accomplish...no one would be taken by surprise.