With lines like these, Hume seems to want to disparage contemporary middle-class society at large. The idea that society is "numb" because "it" is no longer shocked by such "art" is a rather empty analysis. While society may not have been roused into some kind of collective reflection on the shock value of such expressions (not that it ever was so in the past), elements of contemporary society were roused enough to state that this expression was hardly art, and was quite irresponsible.
Fine that the artist wants to believe what he did is artistic expression; it does not mean that anyone else is lacking is understanding by disagreeing with his assertion.
Oh, and art is a very risky form of investment.