I'm not sure where you reached the conclusion that you can't have a realtime public threaded conversation on Mastodon; I do it frequently. The way it generally works is that clients will auto-at-tag people in the conversation, which makes sure the message is routed to all in the conversation within more-or-less milliseconds.
Auto-at-tagging doesn't scale to dozens and dozens of actively-engaged speakers, but neither does human attention, so that's not a problem that needs to be solved.
Auto-at-tagging doesn't scale to dozens and dozens of actively-engaged speakers, but neither does human attention, so that's not a problem that needs to be solved.