It's not that. It's the rich will have untouchable anonymous power. There is no way to stop a cabal of people from taking permanent control and reversing transactions discretely.
I think you were already answered:
Wealth PUT AT RISK PROTECTING THE NETWORK = stake.
As soon as the cabal that holds 51% of the stake in PetkoCoin starts cheating, the trust in PetkoCoin will fall, a new currency - CBeastCoin - will appear (probably with same source code), and the cabal will lose the real-life wealth they have invested in PetkoCoin. This is what guarantees that the cabal (if any) will not be malicious. Of course, it is out of question that the currency software should be open-source (IMO).
First of all, there is no way to prove a secret cabal exists because they would use darknets and other layers of obscurity. Second, they would not just start reversing transactions randomly, they would target their victims very selectively, such as corporations they wish to destroy or buy. Finally, PoS has no way to show evidence that an attack is happening. There are no mining pools to monitor their hashrate. The attack can come out of the blue without warning and leave no trace except the devastation is causes. They will have plausible deniability because the victim will have no evidence that they didn't try to double spend fraudulently. It's the perfect crime.