"I think that there is a secret tragedy alluded to in the limited variations of Frank's melodies: I think that because he realized he was limited musically, and he so wanted to change the world through music, he retreated to the comforts of money. I think that's a key reason for the score the way it is, apart from the need to unify a work of art."
That's a fascinating theory. True, it doesn't jibe with Charley's continual insistence that Frank is an epic talent who just keeps selling out, but maybe one interpretation of the show is that Charley is deluded about Frank's talent -- perhaps because, in one interpretation, he's in love with him????