Nobody actually paid for WhatsApp. At least, nobody I know off. They supposedly required you to pay 1€ a year after the first year, but everybody kept using it for free forever. I'd say they never made any money.
People did pay. Its just that the charging was somewhat selective. Obviously you and your friends weren't in the group that was selected.
Originally, it was a paid app ($1) on the iPhone. Eventually, that was dropped in favor of using in-app billing to actually charge the $1/year on Android... but this was only enforced in a few selected countries at first. Once the Facebook acquisition happened, this effort was dropped. If it hadn't happened, then charging users would have expanded gradually over time.
I can tell you with confidence that very few users in eastern Europe would pay even an euro for an app that used to be free and has free alternatives.
In fact my friends and I discussed that exact particular scenario when we once got a notification from WhatsApp about future payments.
I mean, let's be honest, it's just a chat application. It is almost trivial to build and maintain. And since it uses your phone number, there is no friction to switch to another app--if they decided to charge for it, Telegram would eat it alive in months.
I'm interested to know what you've made or helped to make that is more complex and more difficult to maintain than whatsapp.
But not on iOS, they had to pay.