That's nice. But choosing to believe nonsense won't make it true. The United States of America chose to believe that torturing people is an effective means of securing reliable intelligence. Because that's how it works in Hollywood movies, so how can reality be different? But of course the "intelligence" they obtained this way was not in fact reliable, because a person being tortured doesn't magically know the truth and you don't know if they're telling the truth, so they'll say whatever they think will make you stop hurting them, which is utterly useless.

The only way you can know if intelligence obtained is reliable is to actually test it. With systems like PGP you get proof. Did Alice send this message as Bob alleges? Yes, the message includes proof so he was telling us the truth.

With Signal all you have is Bob's word as I described.

Signal can't stop the Secret Police from torturing Bob, but they can ensure they don't have any way to know if he told them the truth. If the Secret Police were rational that's enough reason not to bother torturing Bob. But we can't make them rational, for some people just inflicting pain for no reason is their goal.