> these top unis have the money and the capability to double their undergrad student size; they have no problem attracting top talent as far as professors are concerned

Professors are not the only bottleneck that the administration of MIT and company would be worried about. With more undergrads and professors comes more administration overhead, a need for more facilities (including land for those facilities that may not be contiguous with the rest of campus, which creates additional overhead of its own), and housing for the students (with the impact on the surrounding neighborhoods that that entails).

Additionally, allowing your school to grow from 8000 to 30000 undergrads dramatically changes the character of the school in ways that can't just be brushed off as "elitist".

And again: regardless of the reasons they don't want to change, I don't see any reason why we should expect any given school to so dramatically transform itself just because college became the default path for the middle class.