I predict that macOS machines will be able to run OpenGL applications for many years to come.

Even when macOS no longer ships with OpenGL pre-installed, you will be able to get OpenGL from Khronos (makers of OpenGL/Vulkan) or your graphics card maker. Microsoft stopped supporting OpenGL years ago, and this is how Windows users have been getting OpenGL.

There is an obvious objection, which is that graphics hardware makers support Windows better than macOS, because of the larger games market. In fact, there is a deeper issue, that Apple may someday move away from Intel and AMD entirely, to their own custom hardware. (See the benchmarks and marketing around the new iPad Pro.) In any event, there is already a product that implements OpenGL/Vulkan atop Metal. It has a good chance of dodging those issues while providing decent (OpenGL-like) performance.

There is another line of argumentation: There is a huge installed base of OpenGL applications, including many technical and legacy applications. Apple would be abandoning numerous sub-markets if OpenGL really couldn't be run. Of course, it's Apple, so who knows what they might do.

Edit: As for rewriting Spiral Knights to use Metal directly: My impression is that Spiral Knights uses a third-party Java graphics API as an intermediary. If that API has a Metal version, then maybe Spiral Knights could take advantage of Metal. If not, then such a rewrite is very far beyond the resources of Grey Havens. (Disclaimer: I'm not an expert on Spiral Knights' architecture or Grey Havens' resources.)