Zulip didn't just "copy [Slack] exactly". The UX is much better than Slack, in my opinion. It's faster, and it puts the conversations center stage. With Slack I always felt that I had too click too much to get to a certain thread, and then it only used < 50% of my screen to show the conversation.

Zulip > Slack, even without this new change that Slack is dumping on users.