How many of those users were actually paying at the time? IIRC they allowed 1 year of free use (even more depending on your country). I don't know what the churn is going to be the moment a currently free customer becomes paying, but it is definitely non-zero. I really don't think that is evidence for a long term sustainable business model if others exist with zero total subscription fees.
> I’m not saying that the ads model doesn’t have its place, far from it, I’m saying that WhatsApp could have lived just fine without it.
The biggest reason for WhatsApp's success was the cost of SMS messaging most international markets. That's actually declined significantly since its introduction, so that's another drag on their pricing power.
Could they have lived? Maybe. But I doubt they'd have 1.5 billion users like they do now, and I don't think they long term trajectory for the business would look sustainable charging $1 a year.