Whatsapps whole schtick from the beginning was something "you pay us $1 pr year and we provide you a top tier messaging service without ads and without analyzing you data or your social graph or anything".

I was enthusiastic about it, and possibly even more enthusiastic about it as I finally got my first invoice (yes, they didn't start billing right away, but they were very open about their plans, unlike Telegram who always just said something like "it isn't that expensive anyway, someone is shouldering it and we have a plan").

It had a nominal $1 per year fee, but in most of the world (like here in India, where it's now practically a utility), they never charged anything because even a trivial sum like that would have caused a ton of friction. That's why it became so popular.