Re: Don't throw out the baby with the bathwater
I think the main thing here is the conflation of flat vs 3d with abstract vs skeuomorphic.
The 3D interface was never completely skeumorphic to start off with. Sure, the buttons were skeuomorphic, because buttons in the real world stick up and then move down when you press them; but I have never had a desk where the paper was below the desk surface, which is where the 3D in Windows 95 et al placed it. Worse, look at the scroll controls in Windows -- you click "up" to move the paper down.
So the 3D interface and its success can't have been down to mimicking the real world, because it didn't.
Thus the 3D interface is not truly skeuomorphic.
The problem with 3D came in Windows XP when Microsoft made icons more 3D. Prior to that, the icons were front-on representative drawings of computers, monitors etc, with 3D implied by lighter lines on the top and left and darker lines on the bottom and right; in Windows XP, all the icons were 3D representations of the ideas with continuous graded shading that made them less iconic, lower contrast and overall harder to process.
Debating "flat vs skeuomorphic" is an oversimplification of the debate, because it conflates icons (which should generally be mostly flat, as they are symbols) with UI elements (which should generally be 3D, as it marks things out as interactive).
The problem, of course, is that this confusion is now universal, because icons in almost all settings act as buttons.
If we look at iOS, the lines are very blurry, because the fact that icons must (Apple rules) fill the rounded-corner-square, means they're visually both buttons and icons.
On Windows, icons have transparency, so they're not visually buttons. So Microsoft invented "tiles" -- i.e. flat buttons to stick your partially-transparent icons on. The name alone tells us that "flat" is a stupid interface: you interact with buttons, but tiles just sit on your wall.