The more interesting application of this IMO is not replacing current popular/practicing musicians and artists but enabling non-professionals who have some musical inclination but limited time/skill/etc. Imagine a synthesizer for dance music with knobs for high level parameters like "grimy-ness" or "shine." Rather than auto generate a complete song with no user input, semantic control could be offered using learned models, allowing a rewarding experience with musical creativity without having to spend a lot of time learning the technical intricacies of these semantics.
Many of the tools you describe are already out in the field. Ableton Live and MAX4LIVE really opens up whole new sectors of tools for those who might be interested. There is a learning curve for music making software though, and it can be rather steep. That said, Instant Haus [1] does sort of what you describe, though it is targeted more for practicing musicians. I'm a fan of GarageBand's "Drummer" as well.
