Apple responds to the Beeper iMessage saga: ‘We took steps to protect our users’::Beeper, like Sunbird and Texts, sought to find a way to bring iMessage to Android users. Its app, Beeper Mini, worked well. But a few days after it launched, Apple took steps to shut it down.
WhatsApp already had a very large userbase when in started charging, and IIRC only charged after the first year of use to better hook people. Given its dominance in certain markets (Europe comes to mind), WhatsApp could probably succeed with a paid model even today, but something with a tiny userbase isn’t going to grow that way.