Also "irrelevant" isn't really the right word as much as "lost". Examining the word completely myopically, things like ability to play piano might seem relatively useless. However, science has recently been revealing that music is a lot more important — to the brain, moods, and the body, even — than we would have guessed it was 50 years ago.
If we allow our knowledge of music to be lost in between the era of musicians and some future rebirth of classical music, then what good have we done?
A good example of the waste that can occur is that of Roman formulations for concrete. Because the information about how the concrete was formulated has been lost, we have spent a tremendous amount of effort just trying to reverse engineer Roman structures like the Colosseum to figure out how they achieve the unique properties that they do. Also consider that the Romans had no way of imagining how important concrete formulations would be in the future. To them, after the fall of Rome, concrete formulations were probably just as "irrelevant" in their minds as well.
I am not so much afraid of my own death as I am that we'll lose knowledge and skill that we as a species have spent so long refining.