Right, in my view it's important that every message have a topic/thread. The reason is that you spend most of your time/effort in a busy team chat tool consuming other people's messages, and that flow is a lot more efficient if you don't have a giant "catch-all" bucket of unthreaded messages about fundamentally different things to wade through in order to find what's important to you.

Zulip, used well, ensures that the messages are partitioned into different conversations, so you can easily do things like skip to the bottom of a long thread to see if it ends in a satisfactory resolution, without missing the unrelated question that you need to answer.