They're working towards a PoS mined coin. This required ETH to mine so only those with ETH can mine. You could call that centralization. ETH will be fairly decentralized. Of course there will be a new coin that comes out and gets better decentralization but ETH will be pretty good.
There will be 1000s of coins that reach a billion $ marketcap with tons of users. This is where the true decentralization comes in, when there are 1000s of different coins distributed through the world.
You can buy the Ethereum during the PoS stage and mine it yourself. Then it will not be centralisation any more. It is that simple.