Exactly. There's no guarantee that PoS wouldn't introduce centralization. Small holders may have various economic incentives to group together, for example, to reduce expenses and increase profit. Capital investments may also favor larger stake holders that will bring higher profits. It will be interesting to see how things develop if/when Ethereum makes the announced transition to PoS, but my guess is a certain number of large players will soon appear. In any case, PoS is no magical solution to enforce decentralization.