We have exactly that... in our DR plan, there is a section for how to cope with the 3rd party source control provider being unavailable/compromised/etc. Update DNS for the equivalent of "upstream-git.foo.com" to an internal address, and continue business as usual.

It's like you said, smaller shops probably think it's over-planning and overkill, but we do indeed have 100's of projects that are "mission critical", that might not have been touched in 1+ years.