Ah, ok. So in that case, people would probably be have to be uncontactable by default (otherwise we'd be back to the current universe). Now you'll have to explicitly opt in to being contacted by every new person you meet. This is fine for some, but it's a massive usability tradeoff. Think about all the non-tehcnical people that Signal is intended to be used by, and imagine trying to teach them about Safety Numbers and a necessarily-opt-in contacting scheme.