I don’t think that the issue is that these schools have “fallen” in the last 45 years. I think that what has happened is that hundreds of other schools have “risen.”
45 years ago, if you were a top student and and wanted to go to a top college, there were probably only about 25 schools on your radar (which included the three above).
In the intervening years, many other schools upped their game and rose into the bracket of “top schools that top students want to attend.”
NYU, WUSTL, Duke, Vandy, Rice, etc. were not in that group 45 years ago and they certainly are now… and in some cases are sitting at the very top of it.