I think it's good to question assumptions, because in our new millenium notions of human intelligence (including complex brain functions like musical creativity) will come into sharp focus. There are two lines of fairly intense research on a global scale that will eventually combine - reverse engineering the brain at the level of neurons, and the engineering of a general artificial intelligence.
There isn't a single music centre in the brain - musical creativity (and genius) will be of special interest to science just because it engages and orchestrates so many diverse areas of the brain to somehow becomes much more than the sum of their parts.
For me Bach essentially mastered and exhausted much of the the creative and technical potential of twelve tone equal temperament *at the moment of its inception* - for me this is an exceptional achievement, since we still use this tonality system it is also unsurpassed.
It also takes a very uncommonly flexible and powerful mind to improvise 4, 5 and 6 part fugues, and many other conceptually complex feats that are attributed to him. It could be argued though that this is just being clever.
My own story however isn't that concerned with these things since I am not inherently moved by complexity or mental agiility. I experience the soaring and incandescant sonorities of Bach's music, the kaleidoscopic waves of fibrillating emotion and it changes my life. That's genius.