The deprecation of OpenGL is kind of inevitable on all platforms anyway. It's a very old standard that isn't well suited to modern computing. And the implementation on macOS was very old and barely maintained anyway – I don't think its deprecation is really a surprise to Apple developers. I expect that it will remain in the OS in its present form for about 4-5 years yet, but the need to move is now clear.
The main bone of contention is whether Apple should have supported
Vulkan instead, the open source successor to OpenGL. I don't personally have an opinion on that, but I'm guessing Apple wants more control over its future hardware/software direction.
Migration doesn't necessarily have to be seriously painful. It's possible to use a cross-platform layer to simplify development. But for great performance, targeting the platform directly is the best option. A well-designed graphics pipeline will be a focussed component within an application, not spread all over the code base, and it isn't like any of these solutions are a world apart. It will take some effort to do it well, but change like this happens all the time.
Ralph Wessel BArch
Software Engineer Speckle Systems