There's an privacy consideration to eliminating the user lock screen wallpaper - currently the user's "Background" wallpaper is "private", i.e. not shown until they've authenticated/unlocked.



If for instance the user's thing is to have some saucy background (surprisingly common, if you've ever done a stint in computer repair!) this change could unexpectedly reveal it to the world (i.e. if they're currently using something else for the lock background).



This could be avoided in an implementation of the concepts by retaining the current separate DConf keys and just setting them both when using any new background selection UI, or selecting the user's current "Lock Screen" background for everything when migrating to a single background.



Alternatively, the background selection UI and DConf keys could be left as is, and the "lock screen" wallpaper used for all the relevant places in the mockups.



To be honest, it's not necessarily clear that "There is no place for a dedicated lock screen wallpaper" is true in order to achieve the above design though - it seems orthogonal.