Github is just a node. IF Github dies , people still have the full repos on their computers.That's not always the case with SVN... And most people are too lazy/dont have resources to set up a git server anyway.
There's also stuff like GitHub Issues, which are separate from the actual Git hosting. So even if the source code is available and the 3rd party tools aren't broken, work can still be interrupted.
Nobody denies your blobs are safe and available elsewhere when Github goes down.
It's the infrastructure around those blobs that makes Github so popular. It's your issues and pull requests, your wiki, your connection to Travis-CI that are gone when Github goes down.
That is why people get angry when it goes down.
You can probably blame those teams for that more than GitHub but people do seem to have bought into the idea that we can just jam everything up to GitHub and get it back later.
The Heroku/AWS outages a few years back had far more of an impact, yet Heroku still (to my knowledge) relies exclusively on AWS-East, because presumably the risk-profile doesn't exceed the threshold to justify the expenditures required to mitigate the risk.
I just wish the attackers would be considerate enough to share the schedule for these attacks ahead of time, so I can plan a longer lunch. The rudeness of these attackers is unparalleled. I'm inclined to write them a strongly worded letter suggesting same.