Your view is extremely simplistic and just doesn't reflect the sophistication of any abstract and complex conversation.

Especially if the topic at hand is a new territory with a lot of unknown unknowns. If no-one ever explicitly says that "OK, we covered topic one and now we move on to topic two. Is there anyone who wants to add comments?", then the conversation was not very civil, was not yet over, and whoever organized the meeting was an amateur, and let the worst thing happen: Alice and Bob change topics for no reason, instead of stopping them short.