You should not be concerned, you will always be able to use Blender on your iMac in Windows via Bootcamp.
I ran bootcamp all the time as a developer that owns an iMac 27’’ and it works like a charm, I use windows 10. It’s a necessary evil with Windows owning still 90% of the desktop market.
Blender has always had problems with mac developers, last time I checked there was only one. So porting to Metal is highly unlikely.
I see two scenarios
a) Luck lands a good implementation of Opengl on top MoltenVK (Vulkan based on Metal)
b) Complete abandonment of the mac platform.
(b) is more likely.
Ton is wrong that MacOS has been a bad platform for 3d graphics, sure it lacked the powerful GPU but lets be sincere , only a handful of people end up forking the money for high end GPUs anyway. He is also wrong on iOS merging with MacOS, its quite the opposite iOS is a MacOS variant since day 1 and the driving force of many of its technologies. iOS is essentially a much more limited version of MacOS customised to run on iOS hardware.
Nowdays this is less than an issue because of eGPU , which are external GPUs connected via Firewire plus of course the appearance of iMac Pro. Apple even offered official support with the latest OS for eGPUs , so 3D wise, things have never been better for Apple.
The real reason obviously is available manpower, expertise and priorities. This is not a Blender only problem, mac developers working on software tend to not care much about cross platform support. Surprise, suprise 
Worst case scenario , you will be forced to use Windows more, fortunately Windows 10 has been a big step forward in terms of usability and stability. Not quite MacOS , but is getting closer.
But even in that case, even without Apple dropping support for OpenGL, it was on its way out and at some point Blender will have to stop relying on it because it will get less and less support. Driver wise, OpenGL was always very problematic and not only on the Mac platform.