Anyway, a high-resolution source could look better than a low-resolution source when displayed on a display with the same resolution as the low-resolution source for a number of reasons, perhaps the most obvious being that the high-resolution source might be of higher bandwidth.
If you have a 2k screen, you may think there's no point watching a 4k video instead of a 2k video. Well there is even if you ignore bitrate - because the 4k video when resized to 2k will contain more colour information than the 2k video shown at 2k.
> A display that has enough pixels to show the pixel resolution of the source should be able to resolve all the (subsampled) chroma information as well, should it not?
Yes, but the point is that a display that does not have enough pixels to show the resolution of the source, will be able to show colour per-pixel in the 4k video, rather than colour per-every-other-pixel as it would in the 2k video.
(Chroma subsampling is not this simple in practice.)
With equal bitrates and high-quality encoders the difference should be smaller, or even turn towards preferring 1080p depending on the bitrate.
Ah this is a common misconception actually! I think that the chroma resolution in video is often less than the pixel resolution, so a pixel resolution higher than your screen resolution can still be useful to get the extra colour data.