In Zulip you can make a "general" topic thread, and then move messages to a new topic when you feel like it's warranted. The normal stream (channel) view shows all threads simultaneously. There's also a view of all streams combined. The UI would be more confusing if Zulip had to display un-threaded messages, because then you wouldn't have a way to view only un-threaded messages without threads being interspersed. If you use the "general" topic method you can just focus on that.