This is where the topic editing feature comes into play; any user can change the topic of a message sent recently, and if someone posts something in a wrong topic, it can be moved to a different topic.

Also, after a while this paradigm grows on you and by forcing you to think of a relevant title for your conversation, it forces you to have more cohesive conversations in my view.

And, lastly, it is possible to send messages without any topic (it defaults to (no topic)). :)