This feels like moving the goal posts a bit. Before Signal, literally all the other options were orders of magnitude worse on either the privacy or the usability dimensions. Which is why there was essentially no mainstream E2E messaging app to speak of. After Signal there were multiple. For any system someone creates there will be weaknesses. The question isn't and shouldn't be did they create a system with no weaknesses? The question instead is did they meaningfully improve the privacy of of a large number of mainstream users and the answer there for Signal is Yes.