Discourse doesn’t have any concept of threading, the expected behavior when responding to several users is to quote and reply within a single response. Multiple back-to-back responses is usually regarded as being pretty spammy, hence the setting.
Yes, because topics are flat, so there’s a degree of relevance between them. If a topic branches off too wildly, it’s for the moderators to split out into separate discussions.
Plugins such as no-bump take that concept one step further, preventing the topic from being pushed back up to the top of /latest until another user replies.