You sink a thoughtful argument by taking as fact two demonstrably false premises, that "IQ is shaped by early development" (nonsense, as any honest psychometrician will admit--this is merely the latest dodge of the less and less avoidable fact that heredity counts more than anything else in IQ, and thus in life); and that regarding trends in social mobility, "The question is whether this developing crisis is due to economic barriers or cultural barriers."
The question includes a third possibility, as implied by my first point.
The twentieth century was a period of radical change in Ivy League schools, changing from a clubbish environment full of dim George the II types, to the brutal meritocracy that it is now; this process began well before the liberalization and continues now after this so-called conservative "backlash." What we are seeing is the limits of those "escalators" of social mobility (which I nonetheless agree we need to do everything we can to preserve).
Listen to your Uncle Ron. He's been there.