To add to your point, many people already use other systems. Tablatures are very, very common for guitar. Lead sheets are used in jazz, and they do away with many of the problems of regular notation (notably, the legibility of chords, and different clefs) by only using it for melodies. In a way, traditional music notation is really only used in classical music. And they're never ever going to adopt another system. The people who wanted another system have already split off.