"hey bro, just download this crappy totally trustworthy app and add me just to talk to me and only me!" is a patently ridiculous thing to try and sell people on.

I don't know, WhatsApp won my local market decades ago by not having to pay 10 cents per message. People didn't really care about encrypted chats until maybe ten years ago, and even today millions are using Telegram for their every day messaging. No idea what the security situation of Vibe and Facebook Messenger are these days, but their numbers also exceed the hundreds of millions together.