[jerf]

I answered a slightly different question than I think you think I did. You said "Well, IMHO this is a purely arbitrary cultural artifact resulting from the evolution of the keyboard (first in pipe organs). Since the octave is divided into 12 chromatic tones, you could in theory devise a keyboard with alternating white/black keys." I gave you a plausible non-arbitrary answer for why that particular pattern was chosen. Not why equal temprement is inevitable or why "7" notes in a scale are special, just why the white keys are ABCDEFG as we know them and not some other pattern. You can put more or fewer notes in your scales in stuff we still would call "Western" music without anybody caring or even noticing. (Much good Western music uses all 12 tones pretty freely.)

As for whether it's a Just So story, sometimes that's all you get. However, if it is just coincidence it's a mighty coincidence.