My opinion is "high art" music is where people care about the MUSIC first, and if it sells that's a bonus. I don't think it ever broke Warren Zevon's heart that he only had one "hit" (and, of course, it was his worst song -- proving that the only way to sell is to appeal to the LCD). He would rather have played to 500 people making quality music than play for 5,000 making junk. And I admire people like that. J.D. Souther once said that everyone thought being labeled a "cult artist" (critically acclaimed but not commercially successful) was a great thing -- "until you ARE one for a few years." Well, some people don't care, they're going to stay true to their art. I don't see that John Prine (one of my favorites) has ever altered his songwriting to "appeal to the masses." If you like it, fine; if not, he's not going to kowtow to the masses. God bless him.
I was also thinking about this the other night while reading some research material. People of my generation and before were not the least bit ashamed of having broad tastes. My parents, for instance, had albums by Jim Reeves (my all-time favorite), Hank Williams, Johnny Horton, and Patsy Cline, but they ALSO has records by the Platters, Dean Martin, Gogi Grant, and Don Ho -- and a Glenn Miller record, too! Today you won't find many people who would want to diversify that much. As a result, even people who perform in a person's particular favorite genre won't get discovered. I mean, we see it here on Y!A all the time: "what bands sound like (insert band's name here)?" If people wanted to diversify, the question should be, "what bands sound absolutely NOTHING LIKE (insert band's name here)?"
You stated it quite well: it's the originality that sets them apart. If someone has never heard John Prine or Tom Waits and they ask you what they sound like, you tell them they sound like themselves and no one else. You cannot do that in popular music, because there's a set formula to "commercial" music. Those who follow it want money; those who throw that formula in the trash respect their craft.