That's the hard part for all of this: if you are building an unlock mechanism the first question you need to ask is how you build a UI which clearly communicates to a user that they are likely giving control of all of their data, location/camera/microphone, etc. to whatever they're installing.

The scammers who push malware under the guise of tech support, free porn/games, etc. are effective enough to compromise millions of people and that's before you get to the question of what it'd look like if a government started pushing access for monitoring. How many people might consider installing something which this guy they met in a coffeeshop says will protect their messages from government surveillance? Now consider how many people might have malware installed by an abusive domestic partner, and where control of the device would extend to hiding the existence of spyware.

This is not to say that Apple is acting without self-interest here, only that I think there's really a pretty nasty market failure making it quite difficult to reconcile someone being able to make choices about their device with a fairly high risk of compromise with potentially significant consequences.