http://vukolic.com/iNetSec_2015.pdf For example talks about the issues of mining pools leading to centralization. Which got it mainly from here: https://www.infoq.com/articles/is-bitcoin-a-decentralized-currency/ which describe most mining pools as a threat and hope that more decentralized mining pools surge (Where you can apply the mining pool to big stakeholder issue 1:1 almost).
I can't find the paper atm which was talking about the monopoly problem.
http://www.coinfox.info/news/reviews/6417-proof-of-work-vs-proof-of-stake-merits-and-disadvantages
Does summarize it pretty well though.

If there is a single stakeholder which MAY define all witnesses and then almost do whatever he wants with the chain. Then this is a major risk for the security of the chain. And thus it is not very well decentralized. I don't know what it so difficult to understand with this.

Imagine, someone from another Blockchain doing this to bring down Steem to value his own Blockchain or Steem fork. At the moment this would be even "relatively cheap".