Mattermost is not built for scale. Mattermost chose a strategy last year to become a ‘on-prem’ solution for small teams - which doesn’t make sense to me - but that’s what was chosen it seems based on this comment Is Redis still used?
Thus, Mattermost for a large online web community wouldn’t scale at all OR cost tons of resources to keep it up. Think 10s or 100s of MySQL or Postgres servers (depending on your audience numbers and usage)
I have a question open for exactly this reason Scaling Mattermost for SaaS
Hoping someone will respond with some thoughts, although considering it’s been 3 months and no one has answered your question so I don’t have high hopes.