Just to lay my cards on the table: I'm registered as a member of the US Green Party and have been involved with various left wing activist movements since I was fifteen. I've also been poor, living in a junkie hotel and doing day labor for cash. My most upvoted comment on hn (before I ragequit my previous account (for basically leftist grievances (!))) was one describing what that was like.
Still, I find the intellectual attitude that you've described deeply unsettling. As you've sketched it, the "social justice narrative[0]" is unfalsifiable: what claims does your theory of politics actually make about the world if any given piece of it can be overturned without making a dent in the theory itself? A theory of social inequality can't be correct in general without being correct in particular cases. Refuting (not "deconstructing") a particular thesis is not "intellectual showboating", it's engaging in argument. What else is someone who sincerely disagrees with you supposed to do?
I've seen this happen with depressing regularity: a self-styled social justice advocate will make a claim, and sometimes that claim will get demolished by an intelligent opponent. (Yes, this can happen, and it is a portent for the future of the left that most activists never learn to take a drubbing from a perceptive conservative.) Rather than taking stock at that point, the social justice advocate throws up a polysyllabic ink cloud ("institutional", "systemic" and "societal" seem to enjoy heavy rotation) and jets away. Whether it is true or not that nebulous social forces conspire to constrain outcomes for the poor in the way that the social justice advocate believes, I have no idea why such verbal behavior should be considered convincing.
This is especially frustrating if you actually sincerely think that the left has good ideas about social policy that should be argued for in earnest and implemented.
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[0] The heavy rhetorical weight placed on the word "narrative" also gives me goosebumps. What happened to plain old arguments-- a set of propositions intended to establish (or at least raise the probability of) the truth of a conclusion? I would hate to think it's because "narratives", unlike arguments, are impossible to demonstrate or refute.