For 1:1 conversations I think you're right. Having multiple channels for communication is fine.
Where it breaks down is for group conversations. If Person A won't use Signal and Person B won't use WhatsApp, you can't easily have group communications. And it only gets worse as the number of people in the group goes up.
Where it breaks down is for group conversations. If Person A won't use Signal and Person B won't use WhatsApp, you can't easily have group communications. And it only gets worse as the number of people in the group goes up.