In the past the function of the "phone company" was a paid for service which provided for your communication needs. You didn't have to listen to ads before you could talk to your friends but you did occasionally get unwanted calls.
This medium reached people rich and poor, all around the world.
The challenge here is greed.
One of the interesting topics in economics is the ability to subvert the supply-demand curve by extracting economic value without the participants awareness. In classical economics the equilibrium point is met when the buyer thinks they are paying too much and the seller thinks they are getting to little for a good or service. In the idealized experiment the buyer doesn't have any other choice, nor does the seller have any other customer to turn to.
But in our internet connected world there is an "invisible" (to the buyer) stream of value which is personal information about the buyer. To date there hasn't been a good way for the buyer to see or negotiate that value. GPDR helps that but it doesn't go far enough in some ways.
What GPDR doesn't supply (yet?) is the practical way of enforcing the theft of personally identifiable information for financial gain. So in your example of a hybrid ad model, if you 'opt out' and now pay a fee, how do you know that they aren't still just selling your data? And if they are selling your data to get extra value out of you being a customer, what recourse do you have when you discover it? What risk are they taking by pursuing that path and maximizing their revenue?
EDIT: From the article -- At the time of the sale, WhatsApp was profitable with fee revenue, although it is unclear by how much. (99 cents per year)