If the app cost $1 per year subscription, most people could afford it, but many wouldn't get their shit together and actually do the payment, and then their acc

If the app cost $1 per year subscription, most people could afford it, but many wouldn't get their shit together and actually do the payment, and then their accounts would be suspended, and the network effect would be reduced. And also, WhatsApp wouldn't be making a ton of money. Payment processors take a huge bite out of $1 payments, the service does cost money to run, and it costs more money to continue to develop. A monetization model that's not a subscription allows very poor people/people who are turned off by putting in a credit card number to stay on the system, while monetizing the majority of users for a much higher per-person value, thus meaning that WhatsApp would take in higher total revenue while maintaining their network effect and ability to appeal to new users. You can see why they want to do the latter.