Why something could happen is not the same as it happening. There are a lot of reasons for elite schools to lose prestige, but it's not happening or no evidence. Lesser-ranked schools also have those problems, so elite schools still stand out.
If you're going to compare them to lesser ranked schools, sure. But the feeling is that, if a rising tide lifts all boat, the tide has been falling in this case, and top-ranked schools have a lot more to lose.