In all, I have come to depend on github, critically because
organizations I am involved in would be severely impacted when we
moved elsewhere. Even for GNU Guix - which pointedly does not use
github - would be severely hampered if something happened to
github. So many projects are hosted on github that distribution and
deployment of free software would be severely broken if github went
down. In other words we have built a monster.

Also
software releases distributed by github should be mirrored elsewhere.
GNU Guix, for example, should mirror and fully archive the source
tarballs that github provides.