Signal will be become popular -- if it does -- just like how every other network based app became popular eg. MySpace, Facebook, Twitter, Whatsapp, Instagram, Snapchat. Examples from work... Zoom, Slack etc. First it's used among a small group of people, has a "cool"ness factor and a USP. Then slowly spreads as other people want to be seen as ahead of the curve and want to be part of an exclusive group. So the Hacker News crowd using it is actually a good sign.

What we need additionally in this case more than the "WhatsApp but better" argument is continually making people aware of the incumbent's behavior, since WhatsApp is well entrenched, to cast doubts in people's minds.

SMS lost because it was costly and lacked features. What will make WhatsApp lose?