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 A

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.