> It's not clear it's even possible to effectively monetize pure messaging platforms like WhatsApp
Well, WhatsApp is moving beyond "pure" messaging with payments and commerce (Eg: JioMart integration in India).
Another approach would be to compare with wechat with 1.2 billion active users generating an estimated 15+ Billion in revenue in 2021 (https://www.businessofapps.com/data/wechat-statistics/) without much North America / EU presence and being banned in India.
Thirdly, click to message ads are already a "multi-billion dollar" business with "strong double digit growth" as of 2022 (https://www.cnbc.com/2022/08/07/why-meta-and-mark-zuckerberg...)