So, I think I know what the negative consequences would be if Nietzche were wiped out. Almost all the great works of 20th century literature would disappear too, as a direct result. No Ulysses, no Hemingway. Film noir would probably never have existed.

It seems strange that you recognize the significance like that of someone like Nietzsche on fields like literature, and yet you're dismissive of Foucault, who is read pervasively across the humanities and social sciences--he's a good candidate for the most influential figures in late 20th century culture. You seem to acknowledge this later on in the post, but in dismissive tones. But if the difference here is that you like the influence Nietzsche has had on the humanities whereas you dislike the influence Foucault has had, that this is a rather different matter than the question of whether Foucault has had significant influence.

Another problem is how broad a brush you're painting with: it seems like you have more in mind the nebulous "postmodernism" of political rabble rousing than the actual historical productions of the figures you get name-dropped in this context. For instance, Lacan is often name-dropped here as an infamously suspicious French thinker, but he was a psychiatrist, not a philosopher or "theorist", his involvement with anything reasonably called postmodernism is somewhere between null and deeply contentious (features like the continuing influence of structuralism and the centrality of the subject to his thought buckle with the usual commitments of postmodernism), and a lot of in the infamous developments associated with post-modernism took off particularly as anti-Lacanian (e.g. Deleuze and Guatarri). As a psychiatrist, his work continues to be one of the central influences on, for example, psychosocial interventions in psychosis. One never hears anything about this in the popular scandal about "postmodernism", which seems to have little to do with such business as historical realities.

Besides these sorts of considerations, the concern here seems to be articulated on the premise that postmodernism references some arbitrary inventions of this or that philosopher. But when Lyotard described the incredulity toward metanarratives--the only gold standard we have for a philosophical view we can confidently call "postmodern"--he wasn't inventing something, he was observing something. We are incredulous toward metanarratives: this is a pervasive and deeply consequential feature of our culture. And this isn't Lyotard's fault! If Lyotard never existed, and neither did anyone else bother to talk about how we're incredulous toward metanarratives (although this is a common feature in post 1950s intellectual culture, so to reasonably enact this stipulation we're going to wipe most of the 1950s cultural slate clean), there's no reason to think we'd be any less incredulous toward metanarratives than we are now--we just wouldn't be thinking about it. And given how consequential this incredulity is, surely that would be a dire shame.

This consideration raises the further point: the pretense of the rabble rousing surrounding "postmodernism" seems to be that there's something astonishing about it, as if the rest of culture was carrying on just fine and then these whacky postmodernists turned up with their whacky ideas. But just about everything characteristically associated with the French philosophers people think of as postmodernists is (i) a more self-conscious articulation of themes already developed in modernism rather than a sudden imposition of something foreign onto the cultural scene, and (ii) has analogs in analytic philosophy, literature, and so on. E.g., why are people scandalized by the supposed irrationalism when Derrida says that signifiers don't stand in one-to-one relationship with a signified but rather their signifying role is dispersed across the language and the expressive acts constituting or enacting it, but applaud the epistemological acuity and scientific-minded naturalism of a Quinean who says that meaning is holistic rather than atomistic? Derrida isn't imposing an arbitrary invention on us here, he's working out in a self-conscious way one of the consequences of how we've been speaking about knowledge and language, and in a way which is very much in the air at the time of his writing. Erase Derrida from history and you'd still have a bunch of other people talking about this, so that presumably someone else would have taken his place as a particularly influential source for these ideas. But again, one rarely hears any reference to this sort of technical detail from the relevant philosophy in the popular polemics about postmodernism, which seem to be carried out in a scene with rather different interests.