The better option is to provide a Vulkan/OpenGL shim instead.
Some get adopted, some dropped and others stay browser specific.
Ever have things been this way. It shouldn't be a surprise when they continue to be.
So why stick to something that will at best be a second rate citizen on Windows too?
Also SceneKit and SpriteKit had quite a few Metal 2 improvements and OpenGL replacements.
They also had someone from Disney showing how they are adopting Metal 2 on their workflows.
So that should state to which direction the wind is blowing.
However as you say it doesn't matter, because all major 3D and 2D engines already support Metal, or are in the process of supporting it.
Note that this is mentioned in the article:
Apple says the entire WindowServer is now using Metal, which should improve the fluidity and consistency of transitions and animations within macOS; this can be a problem on Macs when you’re pushing multiple monitors or using higher Retina scaling modes on, especially if you’re using integrated graphics.
In the case of HEIF, MacOS/iOS will automatically transcode to JPEG anytime you share a HEIF image - it's solely being used to save storage space on your personal device, so in reality almost no one will even notice the change, save for the upside of having more free disk space. H.265/HEVC is pretty trivial to transcode to H.264 or whatever else you want, but given how wide support is spreading I doubt people will often have to do this.
Metal on the other hand is an entirely Apple in-house technology.
(1) WebM/VP9 is only royalty free because MPEG-LA et al have not asserted any patent claims against it. Given that VP8 was granted a license in the past means that it is almost certain to be infringing. The fact that Google doesn't indemnify their users should tell you something.
(2) H.265 is supported by over 100+ more companies than VP9 and is already in shipping hardware from Sony, LG, Samsung, Intel, AMD, ARM, Nvidia etc. It is the standard for TVs and other hardware devices, standard for terrestrial broadcast and will soon be the standard for internet as well.
Web browsers might supported HEIF largely because Imgur and other image hosting will benefits from smaller size.
If most developers are targeting iOS and macOS for gaming, it make sense to use game engines, if I understand some developers had shared their discussion in /r/vulkan found it was harder to code in Vulkan than Metal 2?
There are applications beyond gaming, however, and it is still kind of a drag that OpenGL is still the only de facto cross platform API in the space, and that it has miserable support on macOS.
Are you aware of any project that is under development right now? I know it's theoretically possible, but I haven't seen any implementation yet.
Vulkan is not very nice to work with. It's.. ok.
Metal is nicer
Khronos APIs keep being stuck on their C API definitions, it was NVidia effort to create a C++ wrapper that eventually made it into Vulkan, yet the shading language is pretty bare bones.
Similarly with Metal Compute shaders, Khronos had to see CUDAs and C++AMP adoption to see that many wanted something more productive than OpenCL C. However not many boards do support OpenCL 2.2 with SYCL.
Metal is a nice balance.
Apple is doing the right thing here.
Gamedevs use engines anyway