that's the thing: That setting is pure placebo security-theater. There's nothing to guarantee that this setting actually causes notification on all key changes, whether it's on or off.

Knowing that we all have trouble trusting Facebook, we can assume that all this setting does is inform users when their counterpart has a new phone (which in itself is a very slight privacy issue. I might not want you to know that I have a new phone / reinstalled WhatsApp).

It won't inform users when Facebook adds another public key for analytics and it also won't inform when the NSA adds a key through their special surveillance interface Facebook built for them.

That's the issue with all IM services that manage public keys for their users and thus, my original point was that it's pointless to rage against WhatsApp alone.

Worse: Let's say they change the default due to the present outrage: Then everybody will be pleased with them while the actual backdoor remains in place.