GitHub doesn't just host Git repositories. It's the central location for discussions, issues, code reviews, milestone planning, and any CI process like testing or releases. If it's unavailable whole teams can be interrupted.
It depends on how many engineers you have! But also, there are plenty of other functions in GH besides raw git, like Wiki/PR/Issues/test/deploy pipelines, etc. It can become pretty critical.
Git is distributed. GitHub is very much not.