Some Apple engineer said on reddit that it's because subpixel AA is not so useful in Retinas and HiDPI, but slows down processing and complicates pipelines.
So it's part of the move to Metal and increasing graphics performance, even if it means external lo-res monitors will suffer.
Rendering in monochrome is also faster than true 32-bit color, but we use 32-bit color because it provides a better experience to the user who is the ultimate consumer of the graphics pipeline.
I could replace my old monitors, but they still work well with good colour and brightness, so it's not exactly environmentally considerate, or even slightly necessary.
Adding colour has added very little to UX aside from true-colour icons, the UX itself is essentially the same. Colour is used as a theme on top of a UI perfectly recognisable, and much the same as, mono and 4 colour interfaces of the 80s and 90s.
A current dense desktop monitor seems to be around 160ppi, so 8-bit grayscale would be around 280ppi, which is starting to approach a low-end printer.
It's almost as of it's a trade-off and monochrome so laughingly doesn't cut it, that it's a totally contrived counter example. Almost.
Mobiles and tablets have such high resolution monitors nowadays they probably run without subpixel AA at all…
Edit: personally I value simple and precise language. Constructs such as those mentioned in the Wikipedia article may convey similar meaning but the fact that they exist means that they allow for some variation in meaning and color which is unnecessary in this case.
The original statement is clear from context, since the literal meaning is semantically impossible (monitors are incapable of suffering).
Metonym is just a linguistic term. Such terms are not constrained to describe high uses of language by great masters of writing. They are merely names for specific constructs or linguistic phenomena.
A drunken sailor swearing at someone at 3am could be using a metonym just as easily as Wallace Stevens.