Cool. I guess I was wondering if it's so good, slack could build it in a single 2 week sprint (or a third party could just build a slack app/plugin). Creating a near-clone of a well-funded product to tweak one thing is existentially risky, but it sounds like Zulip has actually been around quite a while.
No, for Slack to be as good as Zulip it'd have to basically be rewritten from scratch. Just the speed of the UI means a complete rewrite of the frontend, and Zulip's UX is its strong suit.
