You joke, but during my time at Netflix there were at least two layers of backup communication systems: if slack went down, we would jump to google chat, and if google chat somehow went down too, there was an IRC server and a (hardware) phone bridge. I never touched any of it because I wasn't an SRE, but I did read through the training one day because I was bored.

[dead]

There was no need to abandon perfectly working technology, with pain some people might learn this. Departmental servers, pesky lordly sysadmins, this was a working sustainable system that even scaled to the phone company and US government levels.