How much infra have you seen of the top 100 sites in the world? I've worked on multiple top 10, and absorbing a 2x increase (and the peak is very likely more than 2x) is a very very hard problem that would cause hundreds of pagers to go off and load shedding to very high degrees. There is just not tons of unused capacity lying around in wait at the scale of github. "No one takes chances with critical components" is also very wrong for the simple fact that you don't know which is the weakest link in the chain until it fails.
I have some good experience and I feel bad about state of these things too specially given that a lot of it could have been prevented. What you have here is not a single service, its a system compromised of hundreds of services, possibly without clear ownership for some of them after these many years and reorgs. There is not a single person or group that understands the whole system from technical standpoint and pressure points. It akin to people trying to plug the holes as the water starts getting under pressure from different joints. This duct taping is present in almost all big enough systems, you name them.
> "No one takes chances with critical components" is also very wrong for the simple fact that you don't know which is the weakest link in the chain until it fails.