That seems shortsighted. I’m not even talking about destroying the app by adding ads or selling user data.
They could add payments between individuals, it would instantly become the default method in many countries. Expand it to pay businesses then, via QR codes like they have in China. Make sure you’re cheaper than cards and Apple/Google Pay, especially for small sums.
They could sell business accounts, and allow customers to message any business by searching them by name (without having them as a contact). Give them a url and QR code that opens a conversation directly. Develop tools for pros to efficiently handle customers. Let customers book a business directly from the app for example. Given the reach of Whatsapp in many places you’d have a lot of interested businesses.
They have a place in the phone dock of entire first-world countries, billions of people literally have the app open all day, and they can’t monetize?