Outbox model is not being challenged, I still think outbox model should be the de facto standard, but it can be complemented. I don't think anyone should implement a thing that just does the inbox model, that would be very limiting -- i.e. you would never be able to follow anyone who doesn't do the publish-to-inbox flow on their side.

But I guess you could have a third-party service automatically forwarding stuff from outboxes to inboxes. Is there a risk to doing this?