Chroma Downsampling Lowers Visual Quality

[Magotchi]

Line 337 of imgmin.c contains:

        (void) MagickSetImageProperty(mw, "jpeg:sampling-factor", "2x2");

I find that the quality is noticeably and significantly better if I comment/remove that line (although the size is increased some).

Here are some test images.

graphic made in GIMP, saved as quality 100 JPEG:
original
imgmin output, chroma downsampling 2x2
imgmin output, chroma downsampling off

random image from rawsamples.ch, resized, saved as quality 100 JPEG:
original
imgmin output, chroma downsampling 2x2
imgmin output, chroma downsampling off

random image from Google Images (and ultimately from thewallgans.com, a wallpaper site), resized, saved as quality 100 JPEG:
original
imgmin output, chroma downsampling 2x2
imgmin output, chroma downsampling off

Every area with fine detail is blurred significantly, and colors tend to look less vivid, in the chroma-downsampled versions.