The only thing SMS offered was that the other person didn't need to be running anything. WhatsApp didn't offer that, and also didn't improve on the other messengers of the day.
Essentially, 95% of the reasons WhatsApp was successful were things you glossed over here.
And it still is. Even cheap feature phones often have WhatsApp support.
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.
I reluctantly installed WhatsApp only after everybody around me started using it, or (the more tech savvy among them who already knew how to "open the internet"), switched away from Facebook to WhatsApp.
Everybody who had a phone also naturally had a phone number, and the phone numbers of their friends were already stored on the phone. Nobody outside the "nerd sphere" had a Gmail account 10 years ago, especially not outside the US.
PS: I don't remember ever paying money to WhatsApp, I think that was a short-lived experiment that didn't work out right before the Facebook acquisition.
It wasn't 11 years ago.