I don't think that Blockchain is only a decentralized system, as it was discussed that blockchain can be both a decentralized and centralized system depending on the way it was implemented.
A blockchain can be either centralized or decentralized. It is important, however, that decentralized not be confused with distributed. While a blockchain is inherently distributed (meaning that many parties hold copies of the ledger), it is not inherently decentralized.
The question here is if we really need a centralized blockchain in which everything is controlled by small group of people. They have absolutely different views on how system should work, there is no space for immutability, security, accessibility in a blockchain of such kind. Just some thoughts from this article:
Ultimately, a centralized distributed ledger should be used in any highly regulated industry - such as financial services - to minimize vulnerability.
Bitcoin has showed us that decentralized blockchain is the best way to minimize vulnerability. It is an open-source system that is constantly under attack and is still alive. What will happen to centralized blockchain if someone outside finds a bug and use to make profit? How much time will it take for developers to find a vulnerability?