Replies to FO posts should default to DM or Public scope. Defaulting the scope of replies to one FO post to a completely different, often non-overlapping set of followers for each account interacting with the thread is a recipe for hopelessly broken and useless threads. Public scope would ensure the various sets of followers can contribute meaningfully to the same conversation started by the OP even if they couldn't see OP themselves, whereas DM would prevent other people's followers from being involved at all.
The real answer is to get groups up and running, and deprecate FO all together. Scope replies to messages posted to a group back to the same group by default, and give each user a built in group of their followers. Simple.