it also provides 99% of the functionality most orgs actually care about
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.
What does it need to become a product better than Slack?
It needs to have better features and better support.
What features are you looking for? I'm sure I've seen the Mattermost/Gitlab team browse here in the past and having something actionable would help them get there
I don’t know. As someone making IT decisions, I can really only look at the options on the market and compare them. Mattermost isn’t clearly better than Slack along dimensions that matter to most teams. Its product management strategy seems to be “we have the same features as slack but you can self host”.