I don't think the argument is that most of the people at Ivies are blue blood WASPs. I think the argument is that if you happen to be a blue blood WASP you are disproportionately likely to get in regardless (or in spite of, in the case of legacies and donors) your merit.
This is almost certainly not the case any more. It was at one time, but it isn't. Princeton's recent incoming class was more than 60% what they called "persons of color", a term that included all Asians. Among the people who might be called whites, Jews were easily 20-30%. If you start to filter out Catholics, there just aren't that many WASPs at these places anymore. For extra credit, go look up how many WASPs are on the Supreme Court. How many years has it been this way?
The stereotypes just don't fit and haven't for some time.
As someone who attended an Ivy, it is simply untrue that it's mostly blue-blood old money hobnobbing around.
First, there's not enough of them to fill the classes every year. Second, wealthy people themselves want their children to be educated with other smart kids.