If I conceptualise it as "your device unlock code is now also your master password but instead of randomly generated passwords we're using a key system actually

If I conceptualise it as "your device unlock code is now also your master password but instead of randomly generated passwords we're using a key system actually designed for what we're doing with it" it makes a lot of sense. Is it less secure than a Fully Correctly Implemented set of current best practices for sufficiently* paranoid geeks? Arguably, yes. Is it more secure than what almost everybody was currently doing while also having the absolute bare minimum of friction to get the benefits it does provide? I strongly suspect so. * I really do mean 'sufficiently' rather than 'excessively' here.