(Nit: The GitHub wikis are also git repositories. https://help.github.com/articles/adding-and-editing-wiki-pag...)

But anyway, the same argument could be made of an asteroid hitting your own datacenter with self-hosted GitLab instead.

Those can be extracted. Way more relevant imo: Links to your repo. For a mildly successful project a key part are external links pointing to your project. Those will point to a wrong location and you won't have control.

IIRC GitLab can import GitHub issues straight from the UI.