Flat interfaces is nothing but regression.
It has existed since the very first GUI. Think about the mainstreams : Windows 3.1 and Mac OS 1. They were definitely flat. Skeuomorphism never existed in that time, because we didn't have the tech to make it.
Oh, really? Here's a shot of OS 1.
It's not much, but they were doing everything they could to connote texture, depth, and realism:
* Textured window header bar.
* The double bar separating header from window content.
* The raised pencil-on-paper icon.
* The dog-eared text file.
* The slight window shadow at bottom and right edges.
* The trash can appears round.
* The disk icon is a literal representation of a disk.
Skeuomorphism came from the start, limited only by hardware, and was required to inform the user. It grew from the very start, all the way into a three headed troll named Scott Forstall. (Yeah, that last bit was just for fun.)
Anyway, I'd argue that the primary limiting factor has always been display quality, against which the tenets of skeuomorphism have raged for decades. The iPhone 4's retina screen blew the lid off in a big way. At the same time, the vast majority of end users now understand how to use computers so we don't need obvious design affordances to inform them. That's why games like Letterpress
feel even more beautiful than they look; decades of design clutter has been stripped away to leave nothing but content behind.
If you ask me, it's like that moment when someone turns off a really loud fan you had forgotten about for so long: immensely, and surprisingly relaxing.