Unfortunately, unlike art, we can't ignore architecture or UI design.
As OP shows, iOS hinting at more content by partially showing the next item is a good example. You can very well create depth and hierarchy with minimal visual cues.
However, what has increased since flat design became the hottest shit ever (2013/2014) are downright malicious interaction patterns, no understanding of the platform (e.g button vs link), and an ever increasing lust for engagement. The design flavour is merely coincidental.
It's a win if the depth and shading was just noise, before. If it was signal, well, now you've got to find something to replace that with, or you're harming UX. And you've got less "bandwidth", if you will, to work with.
Option this, Ctrl that. Yes, they can be discovered socially and informationally, but that goes against intuitiveness.
Simplicity goes too far on the balance of, and in search for, advancement of design. Form + Function. Not an industrial beige box bucket of parts and not a confusing 2001 monolith, but something in the middle.
Now that I’ve said that, I do want to make it clear that I haven’t seen an excellent exemplar of flat design. I remain optimistic though.
The single largest problem is that when we took 3D buttons away, we got no icon for the idea that you some object is clickable. An icon does not need to be skeuomorphic, just unambiguous and easy to recognize, besides, it needs to exist to be useful. Flat design uses a high-contrast background to encode that, what is extremely ambiguous, but there's nothing prohibiting people from creating a better icon.
UIs should not be so plain and spartan that they are literally unusable sometimes. Every Single Button should look like a button, with way more detail than UI buttons have ever had before (why not have a ridiculous number of colors in gradients to make buttons truly beautiful?). When something is highlighted, I want it to be obvious and, again, beautiful (why not glow with photo-realistic lighting effects, for example, since we clearly have the ability and can spare the processing power?).
Finally, the perfect UI (and modernist building) will have no differentiation at all. An exercise in pure, platonic intellectual/aesthetic navel gazing.
You are starting to see the post-modernist reaction in some places in web design, but, as with architecture, I anticipate it will play with forms out of boredom rather than do the hard, self-abgenating work of drawing out the good ideas of the past and humbly driving them forward.
Perhaps I am too cynical.
[1] - https://www.goodreads.com/book/show/41001.From_Bauhaus_to_Ou...
Windows 95 UI: It might be fuggly, but you know where the bodies are buried.
IKEA: We will show you how to assemble this sawdust into a crappy bookcase that doesn't sit square or level using line drawings and language inconsistently.
FlatUI: Physical products should delivered as a white box inside another white box without instructions. Tech support is an unnecessary expense.
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There was once someone who knew.
It did have a useful purpose at one point for expert users, technicians, and engineers.
A large group came along chanting "What does it mean?" louder and louder until it was a thunderous war-cry.
The one who knew was trying to shout the answer but he was whispering in a tornado.
Then, someone else said loudly: "I don't understand. Let's just get rid of it."
Most everyone said: "Yeah! It's useless!"
No one listened and now that that understanding is lost like the Antikythera mechanism and Damascus steel to the sands of time.
Revell must have super clear, most hand holdy instructions possible, for a process that is long and finicky and they assume people will patiently go all the way anyway.
And that’s not just model kits, other Japanese company take the same approach. Finicky and painfully long but clearly explained.
In comparison IKEA spent decades to make shorter, clearer to assemble furnitures, so they can get by with less explaination.
Current day IKEA furniture is way sturdier, easier and faster to deal with than any other flatpack company I’ve seen, Japanese companies included (and I went through a bunch recently…)
A friend of mine posted on her Instagram about a cabinet where she thought the drawer slides were messed up and she couldn't fit the drawers. I looked and just kind of guessed, "is it upside down?". Yep, that was the problem. Apparently the instructions or parts were unclear enough that it was possible to put the feet on the wrong end or something.