He links to an essay by Patrick Dineen, who asked students how many things from Western Civ they knew about.
I went over Dineen's list; I count 21 questions (e.g. who was Saul of Tarsus, have you read Paradise Lost). I'll grade myself harshly--if I knew roughly what something was but hadn't read it (e.g. Federalist papers) that's a no. Reasonably bright, voracious reader, with no TV, lots of books in the house, and not many nearby friends--I knew about 15 off that list when I entered college. (And also some things from STEM that Dineen apparently doesn't think are part of common culture--but they are.)
ACS was populated by children of embassy staff, aid workers, missionaries, visiting engineers--fairly right-side of the distribution. Most of my senior class wouldn't have done half that well on his list--say 6 items or less (there were exceptions). Everybody in class was doing an equivalent of the Grand Tour--which is an education hard to ask yes/no questions about.
My sample is from 43 years ago, with a less flexible curriculum (4 years English/Science 3-4 Math, Language, Social Studies, PE--that sort of thing). Perhaps there has been, as he asserts, an effort to dry up our connection to our past--some replacements leave me glumly suspicious--but it would be tough to prove it by my senior class. Or, as AVI pointed out, by Lewis. A long checklist will show us all badly wanting. like those "100 books every literate person should read" lists
Perhaps it is inevitable that the endless writing of many books(*) will attenuate any core. Among the waterfall you'll find books of equal caliber with the ancient ones. Over time, and certainly while they're in fashion, these new books will have great influence too. So why pick the old ones? (**)
I think "for historical reasons" is precise and accurate. If we are to be of the West, with any confidence that being Western is a good thing, then the history of Western culture belongs in the core of education. But St Johns College spends 4 years learning the core--that's way too much for the rest of us. How much can we pare down and still keep a sense of the flow of it all?
I think we can get something, but don't expect more than a sampling--Dineed asked for too much.
I know the arguments that we are now a multi-cultural society and the curriculum needs to reflect this. I get the claim, but I deny the strong version of it: we have to put first things first. Other cultures should enrich, but not supplant--I really don't want to live in the Balkans. Or, I gather, California, which seems to be halfway there already.
(*) guilty of writing but not publishing
(**) Maybe for the footnotes, he said: not entirely facetiously. Aristophanes' Greece is pretty alien to us--different values, different tribal markers, etc. The Great Books are multi-cultural already!