We could have a world where Vulkan was the one stop shop graphics API anywhere. Instead, we are in this situation where everything but Apple products supports / will support it.
Why did it have to be this way?
[1] http://www.anandtech.com/show/8116/some-thoughts-on-apples-m...
[2] (slide 20) https://www.khronos.org/assets/uploads/developers/library/20...
I only ever programmed with OpenGL 1.3 and 1.4, so my knowledge is not very up-to-date to say the least...
The architectures are all based off Mantle, but OpenGL 3.3 and DX10 were very similar in their hay day, and porting was still fairly limited (you can still partially blame the horrible adoption of the 3.x series that took so long to take off).
Eventually Metal won't be used anymore since there's no point in using the less portable API with the same capabilities. Finally, once use of Metal is low enough, discontinue it in a new iOS/macOS update.
Upgrades with parallel maintenance and deprecation timelines are well understood, and very feasible for a company with Apple's engineering prowess. There's really no argument other than continued lock-in to stick with Metal only.
The pain is going to be that very few video games will be ported to OSX / iOS unless they dramatically increase their market size to justify the heavy cost to port to Metal just for those systems. If Apple cared about customers in this case, it would be an obvious choice to use Vulkan to pressure developers into using the industry standard rather than the Microsoft only product they are using predominantly now.
And sales is what will drive ports.
