Topics are otherwise incredibly useful even with a small number of people, if you want to carry out parallel & wide-ranging conversations on different timescales. Implicitly designing for a single topic per channel forces chats to be ephemeral and makes it very hard to have long timescale discussions.

Eg. If I'm discussing buying a house or a career change (personal) or a new business strategy for my company (work) I don't want all conversations dumped into a single river. Slack's model of threads within a channel feels too schizophrenic; Zulip's model of multiple conversations arranged loosely by theme (and accessible from the sidebar) is much better.

Catch-all topics are good for the ephemeral stream of chatter.

Some might say that chat should be only for ephemeral stuff, but then that is basically avoiding the essential complexity (of long term conversations) which must live somewhere to enforce some Procrustean simplicity on the chat platform.