The whole point of sub pixel rendering is to make text look like it's higher resolution. Once you get to a sufficiently high DPI there's no benefit to it.
Yes. However, the question is "are apple displays "sufficiently high DPI"". They are higher DPI than most other displays - but that is not the same thing.
My answer is "yes, they are sufficiently high DPI."
a) if it truly is "retina," then your eye can't distinguish pixels smaller than it, so subpixel text is meaningless.
b) Laser printers started at 300 DPI, at 1 bit of color and even now have pretty much settled on 600 DPI, still usually 1 bit of color. Retina displays are about 300 DPI with 8 bits of color (in black and white). So, roughly comparable to a laser printer.
You're forgetting movement. Which is not an issue with printers, but is an issue with displays. Such as when scrolling and such. You can detect the difference between moving at 5 inches/sec and, say, 7.5 inches/sec quite easily.
With no subpixel rendering, assuming a framerate of 60fps, you can move only at the following speeds: 5 inches per second (1 pixel / frame), 10 inches per second (2 pixels per frame), 15 inches per second (3 pixels per frame)...
If you're running at 60 FPS and don't mind judder above 30FPS (hideous, but some people apparently don't mind it. I am not one of those people. On a related note: for some reason PM occasionally tears webpages when scrolling, and it drives me crazy), things are a little better, but not much. 2.5 inches / second (1 pixel / 2 frames), 3.75 inches / second (1 pixel then 2 pixels, alternating), 5 inches / second (1 pixel / frame), 6.25 inches / second (2 pixels then 3 pixels, alternating)...
You cannot maintain a scroll speed slower than 5 inches per second (approximately: 300dpi @ 60fps. If you compromise for 30fps, that's 2.5 inches per second instead) without dropping your effective framerate without subpixel rendering. This is an issue all over the place.
As such, yes, subpixel rendering is still required even with retina displays.
Yes, you need the ability to draw content at fractional pixel offsets to get smooth motion. That's different than the "subpixel rendering" the original post was talking about, though, which is a way to draw sharper shapes (usually text) by addressing the red, green and blue "subpixels" individually: https://en.wikipedia.org/wiki/Subpixel_rendering
Except that they are functionally identical. In order to draw at subpixel offsets you need subpixel rendering (or rather: if you have drawing text at subpixel offsets without proper subpixel rendering you get really bad artifacts)
Your claim that you get really bad artifacts on a retina display when you draw lines properly is also needs citation. I don't know if the retina display has enough pixels to meet the claims either, but it seems you're confusing anti-aliasing with subpixel rendering.
It is possible to show, however, that if a scene has a band-limited detail property and the join response of your visual system, your eyes' point spread function and the pixel geometry resembles a Nyquist pulse, then indeed there is a density (which would be equal to the density where 1 pixel is covered by 1 sampling element in the retina) which is sufficient for perfect reconstruction of the image, even if it is moving.
All you need is regular antialiasing, not subpixel antialiasing. To be precise about what I mean: drawing a black line on a white background with regular antialiasing will produce only black, white and gray pixels. Subpixel antialiasing will produce colored pixels. What I'm saying is that you don't need the colored pixels to produce smooth motion.
Apple displays aren't particularly denser than those of other manufacturers. Which stands to reason, as they don't actually make displays. They purchase them from the same market other vendors do.
When the (whole) pixels are already small enough to the extent of being almost invisible to the naked eye, there's no point in trying to squeeze more resolution out by using individual subpixels.
The whole point of Retina displays is that the pixels are below the level of the average eye's ability to discern them individually. That's where the Retina monicker comes from. Asking for a citation for a well known issue, that's been widely reported for years, isn't going to win you many friends.
