Earlier quoted context omitted.
Gtalk came with the original Android 1.0, and you were already signed up with your gmail account when you set up your phone with OOBE.
Could you chat with people on iOS with this? Or with people who didn't have a Google account, or didn't want to setup yet another account at all? Being a free and hassle-free drop-in replacement for SMS was the main appeal of WhatsApp at the time. If there would have been alternatives with the same features, WhatsApp would never have become popular in the first place.
When Whatsapp started out, it was free only for the first year; after that, they wanted a payment. What this experiment has shown, that people did very short term decision without regards for long term.