Git is decentralized as is. It stores all the history locally and sends a copy to GitHub AND/OR any other repo you would like. GitHub is just a part of decentralized git ecosystem. It simplifies communication. But I agree - it is good to use more than one repository. And in case of Bitcoin code it is crucial to use several different private repos owned by different people.
So, what I understand, as Git is already decentralized, we just need to increase hubs to decentralize Github. Right?
GitHub provides an application layer for git. Git alone is a distributed Version Control System, VCS. It is not able to provide either teamwork or public repository environment for open-source projects. What we need is a decentralized application service on git, yesterday.