I have my icons on home screens arranged in folders. It's faster that way, and the larger icons (compared to my computer start menu & quick launch) are good for my imprecise thumb. Your benchmark on a desktop being good is that is provides the same level of productivity as a poorly configured phone?
Problem 1: It's a single-tasking UI (not even my phone is as limited as Win8)
Problem 2: The icons are not well organized (I expect KDE will fix this, where Microsoft never has)
Problem 3: It requires more clicks/touches/actions to launch common programs vs my quick launch, which makes it objectively worse than my quick launch (hard data here!)
Hard Data 2: Windows 3.1 had something close (or at least much closer) to a full screen launcher, Program Manager which was a full window launcher. The transition away went MUCH more smoothly. In fact, Windows 95 still supported the Program Manager from 3.1, but I don't remember hearing of anyone using it or wanting it back.
Hard Data 3: You can get a start menu style program launcher for Android. Apparently people do want it back, even on a mobile screen.