My understanding is those who have more coins on a PoS network have more stake / power. This can matter if there are things like on-chain governance / voting rights, depending on their stake. Those with more stake would also get more staking rewards (it's like an APR % return based on the total staked), and if they stake their rewards as well, then they'd have even more total stake on the network. There also are concerns with those having a majority of the stake in a network being able to disrupt or attack the network (things like slashing based on poor behavior can prevent bad actors from wanting to do that, as they would lose some or all of what they had staked in that case). There are also different kinds of PoS though, and I'm not an expert on it.
Yes, that's a feature or bug, however you see it, that's universal in capitalism, even communism or socialism what implementations I've read about. I guess if one isn't happy with those that have power in one system, a person should switch to another system. It's just so universal that I don't think there's any other way around it other than switch systems as none are perfect, all insofar as I can see are susceptible to the power of consensus.
We've tried to mitigate it with constitutions in the political world and it helps to some extent but many would agree that there is still an exploitable hole in that those with power can use their own to gain more or mitigate risk. And any time you mitigate that feature/bug too much you run the risk of decreasing reward for work and stake, thereby delegitimizing the system itself or in the case we speak of, your currency. So pick your poison.
