Finding self-contained quotes from Sontag and Kael that won't completely distort their ideas has been more of a challenge than I expected it to be, but challenges can be fun, so here's a pretty much self-contained paragraph from an interview with Sontag:
To reproach artists for having an insufficiently radical relation to the world has to be a complaint about art as such. And to reproach art is, in more than one way, like reproaching consciousness itself for being a burden. For consciousness can be conscious of itself, as Hegelians quaintly say, only through its sense of the past. And art is the most general condition of the Past in the present. To become past is, in one version, to become art. (The arts that most literally illustrate this mutation are architecture and photography.) The pathos that all works of art reek of comes from their historicity. From the way they are overtaken by physical decay and stylistic obsolescence. And from whatever is mysterious, partly (and forever) veiled about them. And simply from our awareness, with each work, that no one would or could ever do exactly that again. Perhaps no work of art is art. It can only become art, when it is part of the past. In this normative sense, a "contemporary" work of art would be a contradiction -- except so far as we can, in the present, assimilate the present to the past.
"The Salmagundi Interview", 1975
in A Susan Sontag Reader