Aside from the (alleged) rule that a school cannot pay NIL money, the earnings on endowments remain tax free only if they are used for student aid (it's been that way for more than a decade and a half).
Additionally, in most cases, endowed funds (not earnings on said funds) are earmarked for specific purposes. There isn't really a path for say Princeton, Yale or Harvard to take $15 million or so of endowed funds a put together a football team to compete with the top of the SEC or B1G.
I also would be surprised if any true decision makers at any Ivy League school would be interested enough in having a national championship football program in the 21st century that they would be in favor of raiding the school's endowment to chase a title.