>> if a young child can use a UI with minimal instruction, then that UI has done it's job more or less correctly.
True but only if you believe that everyone is identical and has the same needs.
There are other costs imposed to make that GUI to be intuitive and usable by a 2 yr old. It is massively inefficient to someone who already knows what they are doing. It hides away all useful functionality (what they are now calling "advanced").
Windows 8, iPad, Android, Unity and sadly now Gnome are all like plastic scissors. Great for a 2 year old who by their nature dont understand anything. THe trouble is w'ere all forgetting about the brain surgeon who is already aware of the minimal risks of a sharp scalpel but needs that level of control only a scalpel gives him to do his job.
The real problem is it is becoming hard to find anyone left making actual scalpels, as even all the traditional scalpel makers have jumped on the plastic scissors fad.