It makes more sense as a read, than as a write, as it's completely open to all writers, including a lot of spam, and it pulls in most publicly-visible notes.
Having us look for replies in gigantic aggregators doesn't make sense, for an outbox model, as the client could always include an aggregator as a fallback for things it should be able to display, but can't find in the outbox.
It all needs to be thought-through a bit more. We're thinking. 🤔