Downgrade attacks should be difficult. As the article mentions once a client has communicated with another once using encryption all future communications to th

Downgrade attacks should be difficult. As the article mentions once a client has communicated with another once using encryption all future communications to that client will be encrypted. So a downgrade attack would require either spoofing a new client for a user (eg a phone they didn't have before) which is likely noticeable. The whitepaper (https://www.whatsapp.com/security/WhatsApp-Security-Whitepaper.pdf https://www.whatsapp.com/security/WhatsApp-Security-Whitepap...) goes into more detail. That said, they could push out an update that changes the UI to disable E2EE without notifying the user, and that would be difficult to notice since the app is closed source. For this reason Signal is more secure, despite using the exact same protocol.