Originally Posted by Corrections
Jokes aside, I don't consider Cocoa a proprietary API. Cocoa is just NeXTSTEP, which was made open through OpenStep, and used as a standard by several vendors at the time (at least available for Windows NT and Solaris). In fact, even today you can use the Cocoa API without a Mac, because there's GNUstep.
Now you can argue that Metal is so incredibly revolutionary it will change our minds and our way of living, so Apple will sublicense it to Google and Microsoft, and everybody will be using Metal in the next decade. But that belongs to the future, and all proprietary stuff created by Apple fits in one of two groups:
Group A: Proprietary stuff which was announced like awesome, a revolution, something you really need to learn if you want to earn a living as a developer, but that… at the end, it was abandoned and forgotten (think of QuickDraw3D, AltiVec, Carbon, and quite a few more).
Group B: Stuff which most of the times didn't have such a huge hype, but that at the end was a industry success and become an standard (think of the NeXTSTEP API which turned into Cocoa, the Objective C language, OpenCL -not created by Apple but almost- , and quite a few more).
IMHO, Metal and Swift belong to Group A, but time will tell. In the mean time, I continue using technologies from Group B (with perhaps backends to Metal and Swift if Apple really wants to force us to use them).