Yeah, it would be really hard to change all the millions of documents written with QWERTY.

It'd still probably be hard to convert thousands of existing computer keyboards to anything else. Every keyboard at every school, office, home, etc.

And it'd be harder still to convert billions of existing people to another layout. Inertia's hard to overcome, even if another keyboard layout or music notation system had ever been convincingly proven superior than the respective dominant ones.

No, it's not the same. Current music notation is a result of long evolution, unlike QWERTY. Try to invent usable alternative - then you will appreciate the convenience of current notation.

What are you talking there? Qwerty is indeed not that old as the musical notation but it is ~150 years old, and it was invented for the typing machines in order to avoid jams, so it does have some history and evolution [0]

[0] https://en.wikipedia.org/wiki/QWERTY

"In order to avoid jams" is a popular story but it's credibility has been challenged [1]. It may be worth noted that Qwerty was made before people used all fingers and touch-typed---they were likely to use just two fingers and typing speed might be quite slow in today's standard.

[1] http://gizmodo.com/qwertys-origin-story-is-a-big-fat-lie-493... (This is just a first one I found in English, but Prof. Yasuoka referred in the article publishes quite a few articles about early typewriters in Japanese, in which he lists several evidences that "jamming" wasn't the reason.)