Ivies, especially Harvard, Yale, and Princeton, are so convinced that they are the pinnacle in the Humanities that they cannot imagine that students from any other “lesser” university will meet their lofty standards. It doesn’t just end with recruiting graduate students, though. Their TT faculty are also all almost all recruited from the graduate programs of the same tiny subset of universities.
The main way that they gain some level of diversity is by hiring minorities who have already made a name for themselves elsewhere. Either by poaching tenured faculty who have made headlines from other universities, or hiring writers, activists, etc, who have made headlines. These are all hired with tenure, which is why these universities make it so difficult for their supposed “tenure track” faculty to actually get tenure. That is also why most humanities departments in these universities have very few associate professors or, for that matter, not that many assistant professors. Just non-TT faculty and older professors who were hired with tenure.
For example, Harvard’s English department has 27 full professors, versus 5 assistant professors and three associate professors (one who was hired as an associate professor). They also have 21 non-TT faculty. Of the TT/tenure faculty, either they did their PhD at an Ivy, Stanford, or Oxbridge, and/or made a name for themselves elsewhere, and were hired as tenured faculty. There is a single exception among the assistant professors, and it remains to be seen whether they will make tenure.
Among the graduate students in the Harvard English department, it’s all Ivy League, Berkeley, U Virginia, Oxbridge, and lots of privilege. Oh, there is one graduate of Mississippi State, and he had to be a Rhodes Scholar for Harvard to even look at him.
So yes, Humanities at the Ivies+ (as the NYT calls them) are something of an isolated echo chamber, with the the outside voices coming mostly through people who have achieved fame.
However, since the people from these programs are usually highly placed in academic publishing, scholarly journals, and award committees, so even if it is stagnating, that will not have much of a negative impact on the academic careers of the graduates of these programs.