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.

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.