> The only question it might be reasonable to ask is whether these safety number change notifications should be "blocking" or "non-blocking." In other words, when a contact's key changes, should WhatsApp require the user to manually verify the new key before continuing, or should WhatsApp display an advisory notification and continue without blocking the user.

You seem to be arguing that they should be blocking. While I agree that's definitely the best choice from a security perspective, I kinda doubt most users would appreciate having to manually re-verify keys every time someone reinstalls WhatsApp or changes their phone. In this case, WhatsApp decided to prioritize usability over security. You and I are of course free to criticize that choice, but it's hardly a "backdoor".

I'd add to the author's statement above and say another question we might reasonably ask is whether the notification should be on or off by default. While my gut reaction to that "On, obviously!", upon giving it a bit more thought I think it's actually understandable why WhatsApp chose off instead.

They're not designing WhatsApp merely for security conscious people, but for the masses. The average user is unlikely to understand or care enough about this warning to manually re-verify keys every time they see it, especially when the vast majority of the time it's just going to turn out to be the result of something mundane, like one of their friends getting a new phone. Honestly, I could go either way on this one.