GitHub's primary value add to the world isn't that it's a web interface to git. It's also a social network, an issue tracker, a workflow management system, a code review tool, a wiki-builder, a frontend for your CI, and plenty more.
If github disappears tomorrow, all the code will be safe and decentralized already, yes, but most of the workflow of the software world would be significantly stunted for a good while, because everyone would need to migrate to something else (like GitLab) and then take the time to rebuild all of the data and flow that is on Github but isn't captured within the actual git repository.
This comment is spot on, people who jump to the conversation with the argument that Git is already decentralised don't understand where the most value is captured when it comes to collaboration. It's the social artifact, issues, PRs, etc. And the cost of migrating the community of a project/organisation and their social artifacts is a massive cost, carrying the risk of splitting the community and or stifling any other progress to stay relevant.
If github disappears tomorrow, all the code will be safe and decentralized already, yes, but most of the workflow of the software world would be significantly stunted for a good while, because everyone would need to migrate to something else (like GitLab) and then take the time to rebuild all of the data and flow that is on Github but isn't captured within the actual git repository.