Sounds like a great book, and I commend you for your passion and interest in the art. I agree with all that is stated in those quotes. I would only point out that art is, in fact, linear in the sense that what came before influences what will follow; but, I do agree that it is not linear in the sense that what follows is not necessarily better. I agree that Stravinsky and Schoenberg is not "better" than Mozart or Brahms; only a fool would claim that. Great art is, indeed, timeless. However, it would also be foolish to claim that there has been nothing after Mozart or Brahms that is of equal value. What distinguishes great art is not the value of what it attempts to portray (the times and society), but how well it portrays it; no matter how we may feel about the times and society.
BTW, what is the book?
BTW, what is the book?