I don't think I like your system. To start off, it's very limited, and only really works for the past few decades (and maybe not many more). Rap and techno both use the same instruments and techniques with rap (defined as quick rhyming speech over a constant background) more based on improvisational black spirituals and techno more based on rock, blues, and (going back far enough) classical. Similarly, two "metal" groups (lets say Manson and Metallica), which you have lumped together because of the fact that they use the same instruments, have completely different styles, with Manson being more influenced by modern (new wave, metal, etc) and Metallica being astonishingly similar to classical.
Myself, I think of music as kind of a wave of advancement - a person pushes through at one point, and others will follow along, pushing farther and filling in the gaps betweel the peaks, much like what is happening to rock and rap now*. It doesn't exactly organize well, but it seems to fit the situation a little bit better.
*If you think this is a bad thing, I suggest you listen to some Rage Against the Machine and Junkie XL.