Proof of stake literally makes rich richer, thats the point of it - holders can mint new blocks. With proof of work there is actual work involved tol, not just minting money with your capital.
That's certainly true with a very simple PoS mechanism. I don't think that type of PoS is used in practice with any major cryptocurrency (please correct me if I'm wrong though).
The one I'm most familiar with (Cardano) uses delegated proof of stake, so users delegate their stake to a stake pool and share in the rewards. The platform supports, encourages, and enables this for users with only very introductory understanding of Cardano.
Stake pools also suffer a penalty when they amass too much stake (their rewards decrease), so decentralization is encouraged and no one would be able to get, say, 50% RoA by owning of 50% of the Cardano currency (ADA)
Longer answer: The most you can get from staking is an annual 6-7% return on your stake. I don't fully understand how their saturation parameter works, but here's the general idea: Currently a stake pool with .2% of all ADA staked is considered 'saturated'. This means (in my working understanding, I suspect things are a bit more nuanced) that ADA delegated to them beyond that .2% doesn't receive additional rewards. Therefore it's in the delegators' interest to avoid delegating to pools with greater than .2% of all ADA, and oversaturation works against the stake pool operators as well.
What this means in practice, is that this theoretical group owning 50% of ADA could set up 250 stake pools and fully saturate them for maximum profitability. However, other users can also delegate to those stake pools and share in the rewards. While the operators may receive 1-2% more rewards than non-operating users (there's a limit to how much 'extra' the operators can take, but this extra percentage is intended to incentivize running stake pools), at a certain point of oversaturation (if their pools are profitable enough), it would make more sense for them to just add more stake pools to (or delegate to other pools which are under-saturated) rather than let their existing pools be significantly oversaturated.
cardano.org has an insightful blog post: https://iohk.zendesk.com/hc/en-us/articles/900004671183-Chan... and a useful calculator: https://cardano.org/calculator/?calculator=operator
which are both worth looking at if you're interested in knowing more
I don't consider this a problem but it's a fact that PoS works that way.
I do like that cryptocurrency platforms can put limits on "how much" leverage they get, to significantly close gap between potential "percentage" return on investments between wealthy and 'poor', and remove the ability for third parties to operate which give preferential treatment to people with high concentration of total market cap.
If you own 1 billion USD, then sure, you're going to get a larger return on investments (when measuring in USD) than someone with 100 USD, even if the percentage RoA was the same. Because of how classical economic systems work though, these are very grossly disparate. Someone with 1 billion USD may be able to generate 5-20% return on investments in a year, whereas someone with 100 USD would be lucky to get 3% (certainly, larger returns are possible with luck and wise investments, I'm talking purely in the aggregate over low-risk investments, like holding your money in an account which pays interest).
But let's assume the person with 1 billion USD and 100 USD are both getting 1% interest annually; the person with 1 billion USD is still making more money from investments because they have 999999900 more USD which they are investing.
PoS doesn't solve that latter problem, but it can close the gap on the return expressed as a percentage of investment
(Of course that's pretty much the point of the analogy.)
Your cost basis in the underlying coin is by far the largest determinant of ROI.
It’s like the old real estate adage: “money is made on the purchase, not on the sale”.
Fairness of mining has always been virtually irrelevant to ROI.
(Except insofar as accusing competitors of shades of mining “impropriety” supports a narrative — and invariably a narrative which disproportionately benefits investors with an ultra low cost basis in a benefactor to said narrative.)
“China miners, special deals” etc — it’s a trope that’s been going around for about a decade.
Virtue signaling aside, judging by the raging bull market for ICO coins amongst cryptocurrency investors, clearly no one is concerned with fairness whatsoever. These things are majority premined by insiders and/or sold to prestigious investors in pre-pre-pre ICOs (yes, there are “levels”). What happens is the general public gets sucked into these coins then adopts narratives which suit them. That’s what you’re seeing here. Not that anyone cares.
(Disclosure: I’m invested in a true hybrid PoW/PoS coin, and I’m rather familiar with the downsides of both PoW and PoS mining.)
Proof-of-Work miners have ongoing variable expenses to contend with — electricity cost being the major one.
Conversely, in “Proof-of-Stake”, mining is virtually costless, because it's enough to show you have money to then earn money with your money.
(Disclosure: I’m invested in a true hybrid PoW/PoS system, so I’m familiar with the downsides of both.)