Metal is great, so is Vulkan, so is OpenGL ES on mobile, but it does complicate a bit with portability where OpenGL wasn't as big of deal to port to many platforms, especially on mobile. Mobile has largely been an OpenGL ES only platform except now with Metal entering the game. When speaking of portability on mobile, it was all OpenGL ES versions. Yes consoles have always had their own frameworks to get the most out of their custom hardware and for lock-in exclusives to compete and advance their hardware as far as possible for the console lifeline, to sell as many games as possible as the hardware ages as game content is where they made profit.

The bummer about Apple is that they did help fund Khronos/OpenGL ES and subsequently WebGL from that stack that really revolutionized mobile/handheld gaming. They were big in helping open tech like that as well as canvas, html5, svg etc when the iPhone first came out. They killed Flash essentially and brought H.264 to take a big part of what Flash did, video. Part of how they won was using OpenGL ES and a decent rendering hardware device, it was an amazing thing to see in 2007 and changed handheld gaming big time.

Now Apple has Metal, their own rendering kit, that only works for macOS/iOS where OpenGL was easier to then port to iOS, Android, *nix, even use on Windows etc for desktop/mobile games especially. Apple used open rendering to allow easier porting to their platform, now they have flipped and have taken on a big part of rendering on their devices, a big software part that they hopefully will keep well maintained and up to compete.

But really today there are so many great engines that are middle tier in Unity and Unreal for instance that low level rendering frameworks aren't as big of deal to integrate as it is handled by these middle layer engines. Metal being so prevalent on iOS is only because Unity and Unreal support exporting with that as those two engines make up the largest part of mobile games.

There are four major rendering frameworks now with OpenGL, DirectX, Vulkan and Metal, including others like libGCM and flavors of OpenGL like ES/WebGL. This does complicate things for game engine developers, maintenance and portability with four market standards now. On mobile there are now a few instead of just OpenGL ES versions the dominant one since handheld gaming was taken over by Apple iOS/Google Android. Console game development is a different beast than mobile and desktop in terms of rendering frameworks used for many reasons, mainly that the hardware has to age longer and you need to get every bit of performance.

Most game studios now let the middle tier companies like Unity/Unreal/Cry/Source be their engine team now so it isn't as impactful, so studios can focus on game development, design and content. Back when you did have to write your own engine and rendering layer, it was nice to only have two essentially in DirectX and OpenGL on desktop, and OpenGL ES on the major mobile platforms, console development being the area where there are many frameworks and custom ones per hardware device. The slow progress before mobile and new frameworks was frustrating at times but primarily due to hardware progression, currently it is good to advance and get rendering kits competitive again.