Disclaimer: I'm not trying to be the Devil's advocate here, but just wanted to share an observation.
Apple always removes stuff which looks untimely or just plain stupid (headphone jacks, optical drives, USB/Firewire ports, optical in & out, rosetta, APIs, etc).
Always the same outrage has happened, but things normalize then. People, companies adapt, hell does not freeze over, company doesn't go bankrupt.
I feel that maintaining OpenGL & OpenCL felt like a burden to Apple. We all know that Apple likes to control everything from hardware to user interface, and GPU drivers are one of the most notoriously complex, overprotected part of the software stack. In the OpenCL world compilers and other stuff (I don't remember the terms clearly, sorry) also gets in, and makes everything much more complex.
Maybe this move will help them to slim the drivers to the basic "hardware-software" interface level and build metal and related technologies to their own term on top of this relatively simple interface.
I have a feeling that metal can be directly translated bidirectionally and relatively cheaply to OpenGL (and maybe Vulkan and OpenCL too), so at the end things don't become extremely complex for everyone.
Apple doesn't feel that backwards compatibility is strictly necessary unless things can be translated and made to work with relatively good performance.
As a Linux and Mac user for 10+ years, these are my observations. They may be wrong, technically incomplete or else. Feel free to discuss, debunk, or downvote.