Process and explanation
First of all, I assume humans develop without any fatal troubles in the future. Humans will be still alive and live happy life, technology will develop successfully, no nuclear war and the earth will be still ok to live.
Then, I think about how people in 2145 create songs. One possible future is that, because of the development of technology, computer output a song based on input, i.e., one of the creator's roles is how to choose songs to input so that computer can output a great song. It is similar to a google search skill. Some people who are good at google search can select words effectively so that they can get the results they want to know, and some who are not good at that cannot find information they want to know.
This implies that everybody can write their own songs. Do I think professional song writers are going to disappear? No, I don't think so. It's the same theory that everybody now can take photos but professional photographers exist.
I would rather imagine that the development of song writing computer enhances the ability of human song writing. My favorite drummer Jojo Mayer explains why. Human has infinite possibility :-)