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.
I understand that's one of at least several, and that if Slack isn't actively making trouble, looking for alternatives is a switching cost that may itself be prohibitive -- but if Slack is becoming really troublesome, it might be easier than you think.
We could make do without many of these integrations if we had to but the integrations and webhooks are THE killer feature to me. Chat is easy to solve.
What would be ideal from this perspective would be if they could make the integrations somehow code-compatible with Slack, but that's tough. For example, Zulip's topic system is, IMHO, vastly superior to Slack's threads, but every public message is under some topic (within a stream -- there are two levels, not one), everything posting a public message has to supply one, and that alone might make it difficult to reuse a Slack integration unaltered.