It may be that we are often self-deluded about the past, but things do actually get better or worse, both in individuals' lives and in a society as a whole.  A good example of social decline is what happened to much of Europe in the three decades after 1914, so that when Stefan Zweig wrote The World of Yesterday, perhaps the paradigm case of literary nostalgia, he wasn't kidding himself.  Likewise, it would be foolish to claim that much of the American white working class, which used to have secure union-backed jobs in the world's strongest economy, is merely imagining that life was better in the three decades after 1945.