Warhol's postmodernism

Postmodernism is usually dated at the 80’s. But I tjink that the actually it begins with Warhol’s Campbell’s soup (1968). Postmodernism is already there: meaningless signs citing things out of context, a dreadfyl picture of contemporary times. When Warhol devises the portrait of Marilyn (in the early 60's), the revolution is already complete: no more signs, that bear a meaning but icons, that can only be adorated. Warhol was utterly conservative, but often conservatives are long-sighted, often more than progressist artists. Pistoletto, Kounellis, Merz and the others of the “Arte Povera” in 70’s  produced a number of masterpiece, but their relative scars commerical fortune is due, probably, to the fact that their research continues substantially along the lines of Beuys, of “Soziale Plastik”, of an art that is still meaningfyl in its commitment to change the society, and that rejected the pessimistic message of Warhol. Chia, Cucchi, and Clemente, leaded by the genial critic Achille Bonito Oliva, acknowledged that it was no more possible a meaningful art, and turned to a sophisticated mannerism, gaining wide acceptant and colossal  market quotations.