You can't change it from one day to another, but Apple has all the power to add Vulkan as alternative interface to the same hardware, in addition to the Metal API. They can do this at any time, now or in the future, without inflicting pain on anyone.

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.

> save their own customers unnecessary pain.

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.

Or the 4 engines that most games are made in will just add a metal layer. Or you use something like https://moltengl.com/ to make a direct opengl -> metal wrapper.

And sales is what will drive ports.