If the model is followed by everyone, then you can find ALL of the replies in the inbox of the author of the note. Every person replying should have put the note there. Inboxes are not private like postal mailboxes... you can look into other people's inboxes. The problem isn't that the model has gaps, it is that nostr is a herd of cats running in different directions and people love doing things differently -- which is also a good thing, but makes for compatibility issues.