So, in Alice's case, she gets kidnapped. The attackers are aware that she might have a duress code. They tell her that if she enters the duress code, her entire family will be murdered. If Alice lives in an area where authorities are in bed with and inseparable from the criminal element, is she going to take the risk that the authorities are alerted?
If we are talking about a panic code which erases all information on the phone, do you not think that the same bargain can be applied? In fact, such a system might get innocent people murdered and needlessly tortured since, if someone is suspected of having sensitive information - but actually don't - the attacker might get the idea that a duress or panic code was entered which deleted said information.
In any case, it doesn't seem like encryption is going to save people from real harm.