Those with older external monitors might take issue with that.

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.

I'd rather spend the bandwidth on resolution; I'm perfectly content reading text in black and white.

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.