Why do we love music?

why do I like music so much?Because I can actually penetrate into it.

What would music be like if composers could generate it directly out of their heads?

A real mess, I reckon - I don't know about you, but when I write music, I tend to have many, many ideas happening, sometimes simultaneously - then I go off on tangents, try different stuff, realise it didn't work, so try to go back to the original ideas, try the original ideas with different sounds and instruments, improv around these ideas, have more ideas, get writer's block... and all within a single jam session

Would musicianship eventually disappear as no one would have to learn to play an instrument?

I doubt it, as to compose using an instrumental technique, you need to understand the technique - simply imagining the sound isn't enough, IMO.

What if a device could read your mental state and spontaneously generate the perfect suitable accompaniment?

That was what my "AI" project was partly about - granted I had to feed the melody in manually, but the program "read" the notes, and harmonised it using rules that I gave it. It also presented options, where the rules of harmony could be interpreted in ways that might be considered "better". I think Brian Eno was working on a similar project.

Will everyone be trading personally generated compositions rather than selecting from the available professional recording artists?

Instead? I wouldn't think so!