Putting aside subjective taste for the moment, the skeuomorphic elements of the icon and screenshot add nothing of value in terms of usability or understanding.

I will debate this point. Apple understandably chose the metaphor of a time machine for looking back in time at one's system. Nearly everyone, let alone just those using a Mac, already "knows the drill" with the concept of a time machine. Apple, utilizing this fact, reinforced the metaphor with their historically/fantastically derived interface and icon. If the old interface and icon looked as they did primarily to reinforce a widely-understood metaphor, why decontextualize them by removing the main features that identified them as parts of a "Time Machine"?

The thick chrome with metal bolts outlining the old icon is meaningless to many people, including myself.

It seems clear to me, as it evidently seemed clear to Apple in 2007, that the bolted chrome refers to the fantastical image of a time machine. This fact means the chrome is not meaningless. If you have other arguments against the chrome, I remain intrigued, but meaninglessness is not one.

That is what is meant by "excessive." Its skeuomorphism that is added without solving any problem, which can also be distracting to some or many users.

If you do not mind my asking, what about the Time Machine skeuomorph would distract anyone from anything?

I don't mind if someone prefers the aesthetic (I do too in certain contexts), but many skeuomorphic proponents tend to treat flatter design as an inherent loss in clarity or function, which I think is simply not true.

With all respect, I am not "many skeuomorphic proponents"; I am me. I certainly do not treat flatter (even utterly flat) design as an inherent loss in clarity or function, although a dogmatic flat-only approach does shrink the designer's toolbox. Simply browse my website, which I designed, to see that I do not disown flatness. Flat elements, with a few exceptions, dominate my site. If I thought flatness necessarily led to an inherent loss in clarity or function, I would have designed my site differently.

If you want to present a weak skeuomorph, consult the old design of the Reminders application. It featured a charcoal-colored, leathery background with a dubious connection to any useful real-world concept or object. Time Machine was a useful skeuomorph in that its design reinforced its central metaphor, thereby aiding its users in understanding it; Reminders, however, referred vaguely to reality without a well-chosen metaphor to support the reference. Its leather-ity was meaningless.

(Find My Friends had equally meaningless leather, and it too could have benefitted from a better metaphor.)

What — no! It is so important if you are going to critique something that:

One. You yourself take constructive criticism on.

I eagerly await constructive criticism; I simply failed to understand your initial point.

Two. You use those same skills you critiqued with to explore other possibilities.

Again, I am eager to explore other possibilities.

Three. You don’t seemingly make it your top goal to convince others that your original argument is completely sound and still stands.

You misrepresent me: that was not by any means my top goal, nor any goal at all. Later in this reply, I will attempt to explain why.

You are making multiple arguments:

One. OS X / macOS’s UI quality has declined.

Yes; that statement correctly represents my argument.

Two. This is closely tied to the loss of Steve Jobs.

I intentionally never committed to the argument that Apple under Steve Jobs never had any of the problems I reviewed in my article (because that claim would be false), but I did argue that we could pin many of Apple's early software successes on Steve Jobs. Those are two different arguments, and I only made the second one.

Three. This is related to the move away from skueomorphism.

The decline is partly related to the move away from skeuomorphism. But my article never mentions your examples of egregious "decoration," such as Lion's Calendar, Contacts, and Notes apps. I agree that those applications already had sensible interface designs prior to their Lion incarnations (with the exception of Notes, which did not exist on OS X until Mountain Lion), and I see the ways in which the new designs harmed those applications. The designs for Calendar and Contacts, at least, consisted mainly of non-useful visual metaphors. If I never mentioned those examples in my article and I agree with your points about them, how could I be trying to convince you that "my original argument is completely sound and still stands"?

My argument in return is:

One. Not just the UI but the UX has declined, and although related, I think UX is more important. The user experience is more important than whether something is skueomorphic or flat.

"User experience" is not a separate entity from the user interface. One uses the interface and has an experience. So it is impossible for user experience to be "more important." The interface forms the root of the problem.

Finally, I must stress that "user interface" involves much, much more than a toggle switch between "skeuomorphic and flat" (which, even then, is a false dichotomy; the real grossly simplified debate should be "dimensional vs. flat").

Two. This was occurring during the time of Steve Jobs. Excessive skueomorphism was harming not aiding usability.

As I have already illustrated above and in my article, I never argued that Steve Jobs had immaculately clean hands in this matter.

Three. There has not just been a move away from skueomophism, but a move to more fragile user experiences with Apple software. I don’t agree that one simply caused the other. However, I do believe there are other links. Maybe I’ll have to write an article.

What do you mean by "fragile user experiences"?

So please don’t attempt to boil down my points to ‘sounds as if you agree’. It simply dismisses them. I hope you understand why I am a bit cheesed. Looking forward to reading part II.

I genuinely thought you did agree; I apologize if I wronged you. As I hope I demonstrate with my lengthy replies throughout this thread, I would not like to dismiss anyone's points. I appreciate your eagerness to hear more from me.

Hmm... perhaps Aqua lives somewhere in the well-tended broom closet with the ladder we should be on. While not as visually simple as a t-shirt and jeans, in my view it looks splendid and holds up exceedingly well. It felt intelligent and aesthetically celebratory. I certainly would not offer any of the same sentiments for Sierra.

(Clarification: I believe Aqua looked better or more timeless at certain points in its lifetime than at other points.)

I don't think permanence is necessarily a beacon for the value of things. Does everything have to stand the test of time to actually matter?

Not necessarily: I would never rule it out. But timelessness is almost always a lighthouse for my personal ship.