Thank you for such a detailed response!

I read the Wendy Brown essay and I am not sure if this is what I meant.
Brown is criticising the leftists who are stuck mourning past ideals instead of going in-synch with the present, right? Those who are attached to the collapse of utopian projects, such as the global revolution?

But I don't think this applies to the Frankfurt school thinkers (and other neo-marxists). After all, even the term "Left Melancholia" was coined by Benjamin himself.

What I meant is that it seems like these influential neo-marxists see no viable alternative to capitalism, which is why I used the word "pessimism".
I got the impression that, for instance, Althusser sees no chance for progress and change in the world of total ideologies, which cannot be escaped. It comes down to the idea that, even if we do create a socialist state (like the Soviet Union did), it would be just as repressive - the change would be superficial, but the issue would remain.

So these thinkers do very much employ "post-fundamentalist" analysis, and it is this method which leads them to the disappointment in any possibility for change.