On the other hand, as Git is a DVCS even if GitHub went down the repos still live on local developer machines, so it'd be easy enough to move to a different Git hosting provider.
Many people have warned that using these added-value features makes you dependent on GitHub (Linus Torvalds has a kernel mirror on GitHub, but for this reason refuses to use its other features). Migrating all that metadata is hard (is it even possible?).
I'm sure most of these could be re-imported into an alternative service through its API, even if with some loss of fidelity.
(for others: he's also the creator of git-annex, an out-of-band file storage extension to git, and for a long time was the maintainer of critical parts of Debian's ecosystem. He likes to live in a yurt: http://joeyh.name/yurt/)
Disclaimer: I work at GitLab.
That said, we don't use issues, the wiki, etc.
In these roles, GitHub is of tremendous value and not immediately replaceable.
That being said, I think it's inaccurate to characterize the people using GitHub as a CDN uniformly as "cheapskates". Many are just open source developers, often of limited means, trying to expose their work to the largest audience possible.
When you just start your project you may use github as your CDN and hope no one notices, but when you are as big as homebrew using someones elses resources like this is a pretty bad practice.
There are obviously alternatives but the Github UI just makes it so easy to work with others.
https://docs.gitlab.com/ce/workflow/importing/import_project...