Au contraire! The total and complete separation of OS and UI is quite probably the greatest strength of the Unix system.
It is, on the server.
On the client: not so much.
That's why things are headed the Apple way, even in Linux land:
https://en.wikipedia.org/wiki/Wayland_(display_server_protocol)
Both Apple and Microsoft erred greatly by allowing so much vertical integration of their software stacks; Windows and Mac OS (the original "Mac OS", that is) were severely hampered by having to drag around enormous quantities of legacy GUI elements as the computing environment changed.
I can't really comment, as I'm an expert for neither.
At least, Apple (well: NeXT) didn't start with X but ran their own show...
The Linux GUIs serve not only as a fine example, but moreover as the path to the future.
No, they don't.
KDE tried to look like the current version of Windows (only worse) until it got booted from most distros for lack of accessibility-features, a niche where iOS and macOS are exceptionally strong, incidentally.
Gnome is hideous.
I use XFCE, but it's mainly an "app-launcher" for me (terminal, browser, libreoffice, some other apps). And yes, I use it because it uses very little resources.
Most of the automatisms in Gnome or KDE or even XFCE got into the way of my daily work, so there's little use for running a fat desktop like the first two, when it gets more in the way than it helps.
My usage of OS X at home is bit different than what I do at work, so it works out ok.
Windows Phone was not competitive with Android -- this is not because Microsoft didn't have enough engineers working on WP, but rather because the guys who created Android (before Google bought them out) didn't have to reinvent an OS; they simply grabbed Linux off the shelf, and slapped their own UI on top.
Microsoft hung to the wrong paradigms for too long (keyboard, mouse-like input).
And then Apple created a new OS (and a new UI paradigm) and everybody realized how full of **** they'd been.
Google et.al. fell over themselves copying all the aspects of iOS until it looked like iOS.
Microsoft and Apple entered into a patent-sharing agreement, so when you see a feature of Windows Mobile in iOS, it's not stolen, neither are iOS-like features in Windows Mobile.
"Slapping a GUI on top of it" created the kind of UIs that most people associated with those early Android years don't want to talk about. Because it clearly failed in the market, once people could play with iPhones in stores.
A far more efficient software engineering methodology than constantly reinventing the very same wheel. Heck, OS X itself is an incredibly powerful and flexible OS not because Apple wrote it from scratch, but rather because Apple simply took the 3 decades of effort already put into BSD, and concentrated solely on perfecting an Apple-quality GUI to place on top...
It's not that simple.
They have a different kernel (Mach) and they have a lot of their own stuff like launchd, which incidentally integrates GUI and non-GUI components so stuff can work together (e.g. if you connect to a WLAN, all sorts of stuff has to be done, with all kinds of dependencies).
The userland is mostly BSD, yes.
The BSD license is not opposed to this kind of usage. Apple probably had to rebase their in-house code regularly with the moving target that FreeBSD source code is. While it didn't give back much (on the outside), it still needed people working on that code. As a result, more people got exposure to BSD code and more people earned money doing BSD.
After a while, people move on and they often end up at BSD-centric companies again.
Case in point, Apple's Director of Unix Engineering (or what his title was) and co-founder of the FreeBSD project, Jordan K Hubbard left Apple after 12-ish years and is now heading at iX-systems, creating next-generation GUIs for server- client- and storage administration (FreeNAS/TrueNAS/TrueOS etc.pp.).
BSD has always been about "the long arc of time". And, apart from the license, that might be one of the reasons it was chosen as a basis for OS X.
