Makes me wonder why self-hosted Mattermost isn't far more popular...
Because hosting third party software at scale is hard and when it goes wrong it's your fault. A huge part of business spend is managing liability and having a vendor in a commoditized space you can point the finger at is great.
I agree that is how management like to release themselves of liability. Just my own opinion I would manage chat for Disney if I worked there. Each Jabber cluster can scale to 3 million active users nowadays. It worked great at a company I worked for in the past that was not as big as Disney but had the company grown to that size it would not have kept me up at night. I would document it well enough that anyone could keep it running and I would have Mickey & Mini Mouse narrate a video describing how to troubleshoot it. I've been groped by Mini so she gets most of the lines. Goofy would do a brief cameo.