I think it was highly profitable business because before FB acquisition it was very small company (~100 people) compared to user base (hundreds of milions). but FB did acquisition not because of profits but because of userbase to collect more data. So to increase userbase even more, FB got rid of payment plans and made service for free.

Edit: tried to google concrete numbers what revenue was back then, could not find any clear answer, because it was doing some juggling with stocks etc. https://techcrunch.com/2014/10/28/whatsapp-revenue/

IIRC I even bought the app on the app store and then had some kind of "lifetime" plan, easier times

Yep, first year free, then $1 payment, but if you wanted, you could just uninstall, reinstall and it would reset the entire schedule. Acton and Koum really wanted it to be something different than what it is now.