Apple switch MacOS X to Linux? Why?

It's not to be unexpected. Newcomers to free software, especially software written for Unix and Unix-like operating systems, are first introduced to the software through the wildly popular Linux Operating System.

What most of these newcomers don't realize, though, is that Linux the Operating System is virtually identical to every other free Unix-like system. They all share software written long before Linux became popular or software that continues to be portable among nearly all Unix systems.

If you're not a guru or into the marketing frenzy, you don't need to know what the differences are between, say, BSD and Linux. Just know that there are a few differences, but they are mostly the same. Anything that benefits the Linux Operating System benefits Free Unix, regardless of its flavor.

Which brings us to Apple's Darwin. Darwin is based on BSD and a kernel called Mach, which means it's mostly the same as Linux. Apple chose BSD and Mach for technical and business reasons which aren't truly relevant to the Free Unix world because, except for the MacOS user interface, all the software that sits on top of Darwin is the same software that sits on top of Linux.

People suggest (here, here, and here) that Apple switch from Mach/BSD to Linux. Why? Does a short-term marketing gain of using Linux truly offset the reasons to use Mach and BSD?

Since the difference is in the details and the timing, how can we possibly know?