It seems like there's a lot of people in this conversation on HN still operating on a dichotomy between "skeuomorphism" and "flat". But "skeuomorphism" really ought to be reserved for those UIs that have an excessively-strong physical metaphor, tied to the real world. That's a restricted set of interfaces. They've never really been that popular. It's not likely to make a "comeback" and it's debatable whether they were ever popular enough to be characterized as making a "comeback" at all.

Both skeuomorphic UIs and flat UIs are particular points (or small regions) in a much vaster design space and we should not speak as if we are obligated to cycle between those two particular points, because it will become a self-fulfilling prophecy. There's many, many, many options beyond those two.