I think it is not really the amount of ETH one ones, but more like if the public doesn't want a change, then they won't and no matter how much ETH you own, if public decides not to accept it, they would continue to use the old chain. Remember, you can't make a chain disappear, its always there, even legacy bitcoin is there, but we use segwit.
I think it would be quite obvious that there is no centralization in "power", because people still have it. There is this questionable situation, because whenever Vitalik offers a new idea, people accept, so while people CAN reject, I have not seen it, so you can call it centralized in that sense. There is a "leader" of it, and that's risky, but the blockchain itself is decentralized.
What saves the project is its open source nature. If it wasn't because of that, ETH would've been dead by now. Fortunately, the community can "fork off" into a new chain if things go south for ETH. We still have the original Ethereum blockchain (called Ethereum Classic) with PoW consensus, so there's nothing to worry about.
I don't think things will improve for ETH in the long run, especially when it's a PoS coin. Institutional investment companies, exchanges, and whales, hold all of the cards now. ETH will become less-decentralized over time because of this. Even if Vitalik only serves as advisor of the project, he still has a strong influence over it. With big players involved, they will drive the Blockchain based on their own interest. Not the community. We'll see what happens in the long run. Why do you think no other coin can beat Bitcoin?