It does not triple resolution. Subpixel rendering can only be used to smooth full pixel lines - this introduces color fringing on the sides of font glyphs, but one that can sometimes be ignored when the smoothed lines are thick enough to distract.

If you used subpixel rendering to render lines of fractional pixel thickness, it would just appear as a single-pixel line of the wrong color. The same happens if you render line thicknesses up to 1 1/3 pixels wide, but offset to not match any pixel boundary.

At the same time, subpixel antialiasing can only be used with a specific subpixel layout, in the direction sof the subpixels. For regular desktop LCD panels, this means smoothing of vertical lines only, making characters like "o" look funky and uneven.

With that in mind, a 4k panel with subpixel antialiasing does not look anything like a 6k panel with greyscale antialiasing ("super expensive mac screen" is not relevant, apart from display technology differences not related to resolution or aliasing).

I agree, although this isn't as noticeable as it is on a true low dpi screen. In the monitor market anything higher resolution than a 27 inch 4K tends to be ludicrously expensive, and won't be an option for most people. With Windows having pretty functional fractional scaling, I doubt anything higher than 4K will ever become mainstream, since at the distance you are using your desktop monitor, more resolution is of limited value.

> In fact since cleartype effectively triples resolution

Only horizontally (is 4K a double or a quadruple resolution of 1080p?), and only if you don't care about color (saturation, hue).