Most people only have the NSA as an adversary in the minimal sense that they're caught up in a very broad mass surveillance net. It's true that if you're a whistleblower with sensitive government information Signal is not a complete solution for you. But who cares? Almost nobody is in that situation, and people in that situation have access to other tools that actually do help.

Your last sentence is extremely unfair to Signal and dramatically underrates the privacy it provides to ordinary people. It's dangerous to suggest that because it requires a phone number and lives on an app store it provides no useful protection at all, because that implies it's no better than e.g. WhatsApp when in fact it has a much better featureset.