I personally disagree with the worldview of existentialism, but that's another topic for another day.

A good point you raised is 'a lot of modern theory is a reaction to the essentialism involved with eugenics and the Holocaust, so it is running as far as possible from the thinking which gave rise to the Holocaust'. Now, I think that is one of the core problems with this kind of theory. By overreading into what caused the Holocaust, theory has ultimately become anti-science, anti-empiricism, and essentially against the Enlightenment values of classical liberal thinkers like Locke and Mill. I think this is the wrong approach. Instead, we should re-embrace all of Enlightenment liberalism, including the pro-science, pro-empiricist parts.

Fascism was only possible because the people of Europe hadn't embraced liberalism, particularly its values of individualism and individual-level equality, enough, and a bigger dose of liberalism should prevent it from coming back ever again. Of course, a lot of modern theorists are also anti-liberal themselves, being part of the Western far-left (which have long defined themselves as outsider 'socialists' who stand opposed to establishment 'liberalism', a very problematic attitude I think). This, I think, explains their blindness to the most obvious solution to prevent fascism.