The cool thing about Slack (and Discord for that matter) is that it's essentially federated on a technical level. SlackHQ controls all the instances, but the UX is very achievable by an open source alternative. I'm bullish on federated chat in the next few years. It would be awesome to have essentially the same experience as using Slack, but also a server running on our lab's hardware that we can reboot/debug ourselv…
They've got to nail the integrations. Anytime moving to slack alternatives is brought up with people I work with, it always comes down to everything integrating with slack.
We aren't even particularly dependent on slack, but the convenience of a few basic integrations like google calendar make moving to Discord or anything else a non starter.
The integrations, and chat itself needs an open protocol, otherwise it's just going to be more and more of slack in the future.