well the case is that if github keeps going down and becomes completely unreliable you can take your decentralized code and more it to a different provider. Those other providers will also have value added stuff and you will not lose the core of your business/work (your source code).

If it was a pure central authority + protocol you would be doomed to live and die with them. Imagine if instead of git + github you had some source code protocol that only allowed you to checkout the latest commit of some branch. All code history needed to be viewed by connecting to the central servers. Why not make it even worse, you could only view+edit a specific file of the latest commit on your machine. All other files must live on the central server, you are not allowed to build/compile code locally either... Centralization is the process of you giving up control to a "trusted" entity. If they fail you lose everything