Aren't you just describing chat control in its original form, as proposed by the EU?
https://www.eff.org/deeplinks/2025/09/chat-control-back-menu...
https://www.eff.org/deeplinks/2025/09/chat-control-back-menu...
Still, I kind of fail to see how full privacy as a default is a necessity, if and only if it remains a possibility. Furthermore, by using non open source messengers such as WhatsApp, we are blindly trusting Zuckerberg, a random dude who got lucky and rich and wishes to remain on good terms with Trump, to keep our data as safe and as unreachable as he pretends.