Slack has become a platform.
Every developer, and every company that wants to integrate with in-stream communications, integrates and builds on Slack. All of them.
Chat is nothing new and the core “chat” part of Slack has been and will continue to be cloned and even improved upon.
But developers and partners focus their limited time and bandwidth on the #1 platform, and then later, the #2 platform to focus their energy on, usually.
That is Slack.
And Slack will continue to be the platform of choice for internal communications for that reason.
No matter what cool things others do.