> Coyuld anyone summarize why a desktop Windows/MacOs now needs so much more RAM than in the past

Just a single retina screen buffer, assuming something like 2500 by 2500 pixels, 4 byte per pixel is already 25MB for a single buffer. Then you want double buffering, but also a per-window buffer since you don't want to force rewrites 60x per second and we want to drag windows around while showing contents not a wireframe. As you can see just that adds up quickly. And that's just the draw buffers. Not mentioning all the different fonts that are simultaneously used, images that are shown, etc.

(Of course, screen bufferes are typically stored in VRAM once drawn. But you need to drawn first, which is at least in part on the CPU)

Per window double buffering is actively harmful - as it means you're triple buffering, as the render goes window buffer->composite buffer->screen, and that's with perfect timing, and even this kind of latency is actively unpleasant when typing or moving the mouse.

If you get the timing right, there should be no need for double-buffering individual windows.

You don't need to do all of this, though. You could just do arbitrary rendering using GPU compute, and only store a highly-compressed representation on the CPU.