Fandoms, streamers, and influences will build their communities on Discord.
Large companies which are all-in on Microsoft subscriptions will choose Teams.
Google G Suite ecosystem users will try to use the inferior, bland, Google Chat (formerly Hangouts Chat), which they’re paying for already anyway.
Open source developer communities will ultimately migrate to their own self hosted Mattermost instances. Privacy focused communities will switch as well. Which they should.
Where does that leave Slack? Who is Slack for?
How valuable is the idea of an independent, freemium, cloud hosted, messaging platform which isn’t tied in to one of the big ecosystems.
If Slack started to struggle when growth ends, who would buy the company? Amazon?