Metal was never and never will be a replacement for OpenGL, it’s more similar to Vulkan. OpenGL was trying to be more high level, Metal and Vulkan saw that the high level part was pretty much covered with existing game and graphics engine and decided to expose more hardware to optimize performance.
OOP is everything to Graphics. As a matter of fact, the whole evolution of OOP back in the old days during 70s , started with Simula but substantially evolved with Smalltalk and Self was to facilitate a graphical environment. Which brought the invention of IDEs and GUI applications. Most people dont know this because it was C++ that really popularised it , the decision of Windows to use something that was far more GUI friendly and Apple followed in similar footsteps.
And C++ nowdays can go as low level one can get anyway so even the level is not an excuse not to use OOP.
There is a lot to be talked about the design architecture followed by some Graphics API but then it speaks volumes that even on MacOS people will be far more likely to choose something like Unity , Unreal or other game / graphics engines. Medal as I said will be used by someone that wont benefit from a full blown graphics engine and wants something much smaller and lighter or wants to design his own for his own reasons.
The mobile game market alone has a revenue of 60 billion dollars and 50% of mobiles games are made using Unity with Unreal a distant second at 20% , that leaves a 30% for OpenGL, Metal, Direct3D and Vulkan combine but is highly unlikely they go remotely that high together as there are a ton more much less popular game engines out there.
https://digit.hbs.org/submission/unity-engine-a-unicorn-powering-the-video-game-and-vr-ar-economy/
And then, of course, there is the console and desktop platform which make another 40 billion dollars. Especially AAA titles on PC are severely dominated by Unreal.
On the matter of the future of Blender on Mac , thing dont look good, it took Blender ages to port to OpenGL 3
almost a decade after its release and its still a very much WIP as part of Blender 2.8. Blender cannot afford to port to Metal anyway , it just does not have the manpower to do so. I like to hope that support for Mac wont be dropped but taking into account that there was always an issue with finding Mac developers willing to work on OpenGL or Blender , I am not very hopeful. On the other hand the Mac market is small because many of us Mac users prefer Bootcamp anyway as it offers a lot more choices. Which is why the absense of OpenGL wont be even felt by average users.
Developers will always prefer the highest level APIs possible because they are the ones to deliver the good much faster with little compromise on performance. Plus you can always go lower level and optimise anyway.
Bottom line is OpenGL is gone and forgotten, at lest it old popular days, and soon Android will follow suit as well , cause they have the tendency to slowly follow Apple anyway as they did with Flash. Vulkan is already gaining a lot of ground.
The future belongs to the the game engines , especially now with the rise of VR/AR they are more needed than ever.
There are countless examples that people assumed that is not possible for somehing to die and disappear but it has happened again and again, especially in the game industry where so much money is on the table and where things change blazzing fast.
This thread will also be forgotten and Blender will at some point will have to drop OpenGL completely too. It may take another decade and another 2.8 but eventually will happen.
Its a simple unavoidable fact of evolution. Direct3d , OpenGL, Vulkan and even Metal depend on relic ideologies that have little impact in the extremely complex world of modern computer graphics. Some of then can still serve as low access APIs to an extend but they are just means to an end. If someone assumes that cannot be easily replaced, that someone is going to get quite a surprise party.