If one follows the sales "pie charts", Apple's revenue from "computers" (i.e., Macs) has been slowly declining, whereas its revenue from gadgets (like the iPhone, iPad) has been increasing. Just recently, Apple's behavior with the Mac Pro models (long delay before re-design) indicates they don't give such projects the "emphasis" that an iPhone re-design entails.
At some point in the future, Apple may decide that it doesn't want to be involved in the "computer hardware" market any longer. Not saying this is absolutely destined to happen. But it's something that _could_ happen.
At that point, Apple might decide that its "Mac OS" is still a viable and salable product.
If that happens, we _could_ see a boxed version of the Mac OS offered for Intel-based computers (or whatever processor is being used by the majority of computers at that time).
Or, we might see Apple market "motherboards" (designed and certified to work with the OS) along with the Mac OS. Buy the Apple "package", then install into an enclosure of your choice.
Not saying this is inevitable.
But Macs may not be made forever...