They were and sometime last year turned everybody's subscription into a lifetime subscription. I think it's only a matter of time until WhatsApp will show ads. I see more and more content being shared through it - eventually it will rival Facebook.
It is a free service as of now (at least for me, initially they said I would have to pay after an year, but this never happened). Technically, they don't even have a business model till now. What they have is scale. I think they are afraid that a subscription model will make users churn and they would end up losing the scale. The only other alternative is an ad supported free model or a freemium model. I am wondering if a freemium model would make sense for WhatsApp.