Are blockchains truly decentralized when a small central group of developers control their codebase? Sure, you can choose not to use the latest version, but then you're likely not going to be part of the main network.
If a modified version doesn't follow the same rules, it will never finish the initial sync.
And you can still sync with very old clients. Most of the work on the bitcoin client is optimizing validation. Consensus rules are rarely touched.