Reading over this post gives a decent overview of why you might initially think porting MacOS to other hardware platforms is a good idea. Oh, other hardware is so much cheaper, it would offer such great competition.
MacOS should stay firmly where it is: on Apple-controlled hardware. Why? The same reason that Microsoft shouldn’t allow third-party Xboxes to be sold: the only way to ensure a good experience is to manage that experience end-to-end. Apple’s control over their platform has afforded them a user experience and a brand simplicity that is just unmatched in the PC world. It’s worth noting that not everyone values user experience and brand simplicity over the commodified choice that PC parts offer; that’s fine. But Apple will never be able to compete in that world.
Install a buggy graphics card in a whitebox system and slap MacOS on it, and you’re likely to see a breakdown in that ultra-smooth Quartz Extreme rendering of Exposé effects. Which ruins it. The entire MacOS user experience is premised on one thing: “don’t worry.” The Human Interface Guidelines are little more than a hundred ways of saying one message: “worrying is bad.” You shouldn’t worry about the ambiguity of interfaces, you shouldn’t worry about accidentally doing things that are wrong and irreversible, and so forth.
But the only way to produce that worry-free environment is to manage the hardware. Apple has proven that they can compete with low-cost PCs in the form of the Mac mini. They’d do well to resist the siren song of an Intel MacOS port.