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.
People are too hung up on the look of the interface when they talk about skeuomorphic design.
It used to be that on iOS the bottom tabs would get indented when they were selected. That Z-axis intent, that apps would have even when they weren’t very skeuomorphic, that I hope it returns.
Having a physical material-ness like this is the founding principle of Google Material UI.
I kind of get that sense too. Sure, a "button" may be "skeuomorphic", but it's also a perfectly sensible graphical indication of a place that can be clicked, and only a small minority of "buttons" in the real world ever looked like a UI button. The UIs as a whole shouldn't be classified as "skeuomorphic". That generation of interfaces did a lot of other things that were only vaguely related to the real world... we have tabs, but tabs don't act like this. Looking up at my tab bar right now, they aren't even styled as "tabs", and nobody cares because that was never the core part of the appeal. Non-flat design had IMHO a much richer design language than flat design ever developed. Missing that design language and being forced into baby-speak for so much UI, when GUIs are already baby-speak themselves in so many ways (yes, the old UNIX hacker still sees the world this way) is very frustrating.
Even just a couple of layer's worth of visual depth, that ".5D", is so very useful.