We live in an age when the traditional great subjects - the human form, the landscape, even newer traditions such as abstract expressionism - are daily devalued by commercial art.

This quote means that in our modern age, the traditional forms of art such as the human form or landscape paintings and abstract expressionism, which were once seen as very important and valuable, are now seen as less important or valuable because commercial art (art used to make money) is given more attention and respect.