Unlike Slack, Mattermost is open source...
Teams suffer when SaaS companies lose focus. Since it’s open source,
Mattermost can endure through its community. If quality declines,
anyone can fork the code and take stewardship.
Open source and focus are the wrong arguments here.(a) Mattermost is built by a video game company, so there is no focus to start with.
(b) Moving from a SaaS product to a self-hosted open source component runs against the process of commoditisation so is ultimately only ever going to be of niche value. After all, why spend scarce and expensive engineering time setting up, maintaining, and potentially having to fork a product, when you could just pay $however-much/month and not have to worry about it.