Whatsapp is messaging-focused, but I'm willing to bet the quotation you just gave is not even 10% of the reason people choose to use it.
If I understand it correctly, people use it mainly because MMS was a dumpster fire and WA was the first platform which got critical mass in most countries, which it achieved by being both pretty good overall and by being cross-platform.
The encryption is a nice bonus that everybody likes, but you can't prove that is a primary or even major reason why plumbers in India, tour guides in Dubai, and school parent groups in the US all choose to communicate with it, personally and professionally. If anything, I feel like Signal must have by now poached a good number of the people whose main concern is "How encrypted is it?"
Also, Gmail is not a public forum and people don't mind that it's 'ad-ridden' either.
I don't think anybody in my non-tech circle even knows that messages are encrypted. It's just a convenient way to message people and share pictures from android phones. At some point in the past it was viber and before that fb messenger. I know older people who wouldn't know how to attach a document to an email but can share vacation photos via whatsapp, and we have group chats between friends and family. People also care about their chat history, and if they don't know that the data is encrypted and needs to be backed up, they loose it when transferring to a new device. It's happening all the time, a lot of common users would expect chats to just stay in the cloud somewhere and be available.
those are literal public forums people go to expose themselves. you don't have a very good point.