To judge the value of a piece of art, you must consider it's audience and it's capability to effectively produce a desired effect in that audience. Because there are characteristics known to consistently produce known effects across the set of possible audiences, beauty is solely a property of the piece itself.
Woah, there. I don't think so. Paul worries that artists no longer seek those traits known to operate effectively on the global audience, because they are taught that 'beauty is subjective.' Despite this wholly justified concern, beauty can only be justly described as a complex mixture of reactions that are shared amongst members of the global audience, audiences of diminishing size, on down to an audience of one. To deny the subjective nature of art is to deny that experiences change your perception. To deny the objective nature of art is to deny that audiences with shared values exist.
Neither of these denials are valid--beauty is both subjective and objective. Imagine that you can normalize the value of a piece by taking the product of an audience's size and a scalar representing the piece's capacity to effectively speak to that audience. Now sum those products across every possible audience, where an audience is a group of people who respond consistently toa single variation in the character of a piece.
By the definition of an audience, the traits constraining an audience must shrink as the audience size grows. Pieces pandering only to the largest of audiences employ variation in the basest of traits. Hello, American Idol. There must be a set of audiences for which variation in artistic traits produces the greatest value. It's not clear to me that this optimal set includes many, or even any, of the largest audiences.
This analysis discounts art whose purpose is not to be valuable using the equation described above, but some other criteria. In other words, beauty is too subjective to be called objective. What Paul should be saying is that this normalized value of art has dropped since we began to think of beauty as subjective. A valid concern, but an invalid conclusion.