Two largest markets for Whatsapp are India and Russia... Whatsapp knows Indians usually don't have credit cards, and if their service is stopped, it is very easy to jump to competitors like Line and Kakaotalk.
I believe Whatsapp's days are numbered. With Google's unified cloud messaging Babel coming in soon, as well as innovative and free services like Facebook Messenger with Chatheads become more commonplace, Whatsapp has no chance... Competition is catching up very quickly... Facebook messenger and Google Babel would be cross-platform, available on all major mobile OSes...
@OP, don't pay yet... Whatsapp will probably extend your service for a few more months... Whatsapp's strategy is to give their service for free, and when mass of users is sufficiently large to make Whatsapp very sticky and important, then they would go for deals with mobile carriers and have direct carrier billing to deduct their 1$ per year directly from your phone balance... But I believe they will wait... And continue to upgrade users for free...