I wonder if some of the changes made to macOS (née OS X), were made because iOS has freed the mac from having to serve complete novices and very casual consumers, and instead focus more on serving a segment of professionals and serious content producers whose needs are different from casual consumers, and may be better served by a more subtle, muted interface.
The most pointed criticisms I've seen of Apple's UI changes have been from people who work in visual media. "Serious content producers" use Finder for the same things as the rest of us. The interface isn't consistently more subtle; where color hasn't been removed, it's brighter and more saturated. iOS has moved in a similar direction aesthetically, so it isn't a matter of different interfaces for different audiences.
That's certainly a large part of it, but I don't see how that detracts from my point. The touch interface is as much software as it is hardware. To the extent that we forget that, it really proves that Apple really nailed the software. Things like flick to scroll and pinch to zoom seem obvious now because they feel so natural, but they are a feature of the software, not the hardware.
I also don't agree that the touch interface is the only reason an iOS device is much easier to use. Anecdotally, my father in using his iPhone for the first time feels like he masters a computer. Mind you, this is not someone who is a stranger to computers. He has used a desktop PC for years at his office.
The Norman/Tognazzini article points out the loss of discoverability, feedback, and recovery in iOS.
I don't know if I've read that particular article, but I've seen similar criticisms before. And I do believe that there is something to them. But I also feel like they are blown out of proportion and/or fail to take into account the constraints of the small size of a phone screen. Not to say that Apple hasn't made outright mistakes, but I do think they get it right far more than they get it wrong.
The most pointed criticisms I've seen of Apple's UI changes have been from people who work in visual media
That's hard to argue against, since you don't say what specific criticisms they make.
The interface isn't consistently more subtle; where color hasn't been removed, it's brighter and more saturated.
I probably overstated that particular point. There is certainly lots of evidence these days to suggest that Apple doesn't care all that much about the pro market.
The one that says "A funny thing happens when you design a computer that everyone can use. Everyone uses it"? That's in the article.