It goes around in cycles, and “postmodernism” is just the name we gave to the idea of borrowing superficially from movements of the past without submitting to the deeper totalising meanings that the aesthetic/stylistic details in question emerged from, at that point in history. (It was so named because it contrasted with 20th-century high modernism and its belief in overarching systems and the possibility of universal meanings.) Looking back in history, you find postmodern-style epochs of stylistic appropriation/recontextualisation at various times, such as the rediscovery of classical aesthetics during the Renaissance.

It could be argued that postmodernism emerged from an unspoken universalising ideology: the me-first individualism of the baby-boom generation and the market-oriented neoliberalism that took charge in the US and UK at the start of the 80s and proclaimed its victory for all time when the USSR fell (as in Fukuyama’s “end of history”); to wit: there are no universal truths, only individual opinions, all overseen by the invisible hand of the market. Since then, history has loudly restarted, and the eternal consumerist utopia of the 90s feels as retrofuturistic as an episode of The Jetsons. Parts of postmodernism will be absorbed into what follows where useful (i.e. the idea of novels having metanarratives, or sampling/appropriation by mechanical reproduction as a creative tool), others will become a stereotypical period feature, in the way that that art-deco font signifies the 1920s or angular motel signs the 1950s, and the cycle will resume.

Having said that, with history having restarted, various kinds of Romanticism and/or Neue Sachlichkeit-style realism could emerge; though, of course, not in the same form as past versions.