Market manipulation is prevalent in all markets all the time. The risk of market manipulation comes from other participants (the bigger a share of the market a whale has, the less risk he has) and from real world anchors (which are what sets the price of stocks in the longer term despite their short term randomness).
ETH whales always had the ability to do price manipulation. And they did it to some extent mostly to the upside. But they risked breaking the market if they manipulated to the downside.
The miners also had real world selling incentives. The game was rigged but at least price incentives aligned.
The problem isn't about whether PoS works. The problem is that the lack of real world anchors means the whales are free to manipulate price as they please. They can agree as a cartel on a price, low or high, beforehand, and keep buying from retail low and sell high and gain higher and higher share, manipulating both to the upside and the downside.
The problem is that PoS has no downside price manipulation disincentives, and it encourages hoarding. You'll end up like diamonds where most of those are hidden outside the market by a huge whale that exploits a monopoly on supply.
