As much as I like your post, I must disagree with your overriding point which leads into your framework: Level design is not art, and so is removed from the creative process.
Line by line on your analogies-
Painting a painting is creative expression and by comparison, a drywall painter does not draw building blueprints, they paint walls, not create paintings.
A musician and any creative professional does and do care very passionately whether people experience something they create, as again, it is about expressing oneself regarding ideas or emotions instead of just saying or doing something. IF someone just wants to play an instrument, that is then a hobby for which they choose to enjoy alone for interest or learning a skill, typically because they are passionate about it. The issue with that analogy is that when your obsession becomes your profession, it is no longer your obsession.
The pianist, will certainly require mentoring and tutelage whilst they learn how to play, but writing and composing a new piece of music? Yeah, that requires feedback and again, is creative expression - yet there are only 12 notes on the chromatic scale...
Level design is part of interactive entertainment, and not having feedback and just chucking a map on the pile of maps which already exist in the hope that one day one of the maps may or may not be played is a highly questionable process for blocking out a level. I simply refuse the believe someone will hire someone to create maps for them, and never have the map tested, give it to an environment artist to not go over the lines. I really do not think it works like that. Figuratively speaking.
Surely, it is no different than UI?
OT, Level Design is not art, it is design and it must follow the rules of the game for which it is designed for, not the other way around - and the only way to get better at it is feedback. It doesn't matter how pretty I make something, if it doesn't follow the principles of the game for which it is designed for.
I also am a creative writer btw, and have finished everything I began writing - that, I do for pleasure.