You’ve clearly never run an org. Cost is one of many factors that matters. MS teams is also free as in beer when I’m already buying office.

I've run mattermost for a moderate sized team while doing my dev duties, it was never a bother...what specific problems did you have?

I maybe spent fifteen minutes a month on upgrades

I do not want to have a piece of undifferentiated infrastructure at my org to be tied to the employment, enthusiasm and attention of a particular engineer.