Zulip seems to be trying for "a better Slack", down to the (disgusting) pricing model centered around keeping your message history hostage.

I have no need for a slightly better Slack. I can see how your topics make it slightly better to find message, but how do I find topics once there are hundreds of them? How do I find unrelated issues that popped up in a thread, like they tend to do in human conversation, and that people didn't bother to make into a new topic?