> WhatsApp is end-to-end encrypted, for instance, and it's used by billions.

and it causes no end of pain when you switch phones (esp. if you loose one). Of all the chat services i use, Telegram is the only one that NEVER, EVER LOST ANY OF MY MESSAGES. Maybe for some people privacy is more important ; for me, not loosing any message I have under absolutely no circumstance is the n°1 baseline requirement for something to even be called a chat app.