There’s a difference between the spooks being able to read everyone’s messages and the ordinary police being able to do so. Assuming that Five Eyes or similar have a secret way of decrypting Signal messages, it won’t remain a secret if every drug dealer who uses Signal is swiftly arrested. (Even with the trick of parallel construction, postal inspectors magically getting lucky every time someone uses Signal would get suspicious pretty quickly.) If the spooks can read your Signal messages, they are compelled to ration that capability rather than burning it.