If we look at all the various musical instruments and how they produce sound then we can see that this musical instrument is like an extension to the human that is playing this instrument. This may sound like a far out idea but basically people that ate talented can produce music for almost all instruments and it will still sound awesome weather it is played from guitar, piano or with drums. Basically the music that is played can be achieved from any of these instruments and thus is an abstraction. If you look at the music science then you will see that music is composed of various notes that are then coordinated through 8 different octaves and basically is the representation of various different frequencies. If you want to represent frequencies graphically then using music theory you need to use notes and octaves and then you can put down your created rithm.
It is very interesting to think about how musical instruments have evolved from very simple drum like things like tamburini and whistles to famous guitars and pianos. But as we all know technology does not stop evolving and as the computer age progresses more and more music will be produced through digital equipment that allows artists to quickly compose and reproduce their creations. If you think about what music is then it is an artform that expresses that artists frame of mind in that exact moment and because we humans are tool-builders then even in this field there will be significant breakthroughs that will allow us to create music with efficiency and speed that is certainly not available at this point in time. Of course there are those digital DJ programs that allow you to compose virtual music but because these programs run on our computers the input through keyboard and mouse is still too slow to facilitate real time music production. Imagine how the industry will change when someone invents an input device that allows us to control that computer with our mind and you will just have to think about a melody or note and it will instantly appear digitally and could be reproduced and changed. This is of course science fiction but I think that it will become reality in the coming decades and almost all music will be created in that way as it will be more efficient than writing down notes and trying to play the same piece over and over tweaking and improving till it sounds just right.
I also imagine that in the near future guitars and other musical instruments will be able to directly input data into our computer and allow artists to play a tune and it instantly will appear on a digital note-sheet that you can then replay and tweak to your liking. But I am not a musician and because of that there may be factors that I am not aware of that might influence the future of musical instruments.