VGA and Y/Pb/Pr component video are comparable in quality, exactly what quality you get depends on the bandwidth of the electronics for each of the three color paths.
Currently the DVD and ATSC DTV/HDTV formats give the source material half the horizontal resolution for the Pr and Pb compared with the Y of component video. It would not be possible to use VGA (RGB) unless all three video subsignals had the full resolution and then the video would not fit on the DVD or be broadcastable in the channel allocation given.
Meanwhile for DVD (and 480p DTV) the bandwidth for the Y needs to support 720 pixels across. But for the ultimate rendering (seldom seen today) of 1080i HDTV the bandwidth for the Pb and Pr needs to support 960 (half of the 1920 for Y) pixels across.
THe Y (luminance or dark/light de5ail) of S-video is the same as component video but the color has significantly less reslution, about 160 color changes across the screen maximum.
Video hints: http://members.aol.com/ajaynejr/video.htm