I get the AVAILABLE LIGHT => LENS/Aperture => SENSOR => READ OUT SYSTEM, but I am under the impression that there is a difference in image quality, as in how an image looks scaled down on a monitor or in a print, between a smaller and a larger sensor, all else equal, with the size of the sensor as the only variable.
Nope, I don't think so, don't let marketing get you. Unless you're limited by the smaller sensor's issues and benefit from the larger pixels (yes, do try shoot red gradients and postprocess them). A ff sensor doesn't have any "magic", you can either name the problems it solves or the creative potential it adds or there's nothing to it.
Btw, if crop is becoming 'good enough' in good light, wouldn't FF still be better than good enough, and remain so as technology advances? So the threshold would move with the technology.
Fortunately, there is such a thing has human perception. You can only recognize and tell so many colors apart, see only so much resolution (compare a 200ppi vs 300ppi print...). FF will keep giving you an edge in difficult situations like thin dof, very low light or higher shutter speeds, but personally I think the 18mp crop generation is "good enough" already in good light. At some point, the picture content isn't disturbed by tech limits, and that's what counts.
But I see a similar difference between 40D and 600D where the 40D has approx. two times the pixel size of the 40D. Shure, the 600D gives more detail but the 40D has sometimes the "shinier", more realistic photos. More "brilliance" is a good description.
That is even after downsizing 18mp->10mp and proper postprocessing? I know the 40d seems to have hit the sweet spot back then, but I often find people tend to compare sensor generations at 100% crop sooc.