People familiar with macro economics are probably aware that our current capitalist markets tend to centralize the wealth distribution around the world:
As you can see the current wealth distribution around the world is approximately that 10% of the richest people in the world own about 80% of all of the wealth in the world. How does this consider Ethereum you might ask?
Well, when the PoS launches, then to my understanding the validation / voting power is determined how much ETH you own. Also to my understanding, there is no artificial boundaries how much ETH a single person can own, just like there are no artificial boundaries how much hashing power a single node can have.
So how does ETH prevent a situation where 10% of the highest ETH holders will end up owning 80% of all of the ETH, and hence centralize the blockchain? Because without any artificial boundaries, this will eventually happen. It is a statistical inevitability.