People will make frequent mistakes if you put the privacy decision at a per post level. (And not just average users: see stevey's Google Platforms rant)

Having different apps, chats (Discord servers), accounts (at-a-push) for each privacy circle is much clearer to average users. Migrating a whole group of any size to another platform is hard, hence many of us are stuck with Facebookk in case we get invited to something we don't want to miss on it, but new platforms will continue to emerge and some will succeed.