First link references

https://cacm.acm.org/magazines/2018/7/229033-majority-is-not-enough/fulltext
For their assertion that bitcoin mining pools can undermine the network, but I'm not going to spend one cent let alone 8 thousand to see exactly what the "greedy" miners can do to undermine the network, so I'll cautiously point out that the 51% attacks have been possible for years and we still have no proof of any of that happening on bitcoin. Even if it's a possibility, it's in the realm of counter to the incentive of mining/securing the network, and if it does happen it likely will be reversed, new rules put in place and things will ge back to working in no time.

As for the other article, it's complete horseshit. As Vitalik said, has there been any such attacks on POS.
No

And that's 3 years later and still nothing to show for it.

Let's suppose for a moment that there's ANY merit to this:

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".

So theoretically, because of the size of someone's stake, they "can" do whatever they want with the chain, saying essentially that with their stake alone, they could force everyone else to comply. Well, good luck getting them to undermine their position so that your unfounded fears and concerns will be assuaged, because in reality, not only is it a complete fantasy to think that one stakeholder could do such things, but it undermines the entire reason for holding stake to penalize, to tax, and to otherwise undermine the power of the stake in relation to how much it is, thereby making holding a certain ammount of stake a risk without any reward, and exactly like a pool of miners colluding to cheat a few bitcoin while essentially destroying the value of bitcoin it's the same for one large stakeholder to invest into the system only to undermine it, because sinking one ship in your head will raise the level of the ocean.