Integration happened. And these public colleges and universities were no longer able to legally deny admission to the children of taxpaying citizens who supported them. Private universities couldn't ban admission to students based on race either.
About your second point. Celebs and philanthropists can't keep bailing out failing or poorly managed schools. Some of them are money pits.
That's not the problem because they can still legally prefer Black students. They take White students because they need to pay the bills. If Black enrollment at HCBU's was on point then they wouldn't be needing to take White students. And I don't blame the students - when I was a kid there were very few HCBUs whose name was being put at the same level as the good PWIs out there, and none even close to the top PWIs. Even if they might have advantages over PWIs for many reasons, that's not the narrative most kids hear, especially if they're not growing up in the South.
For HCBUs to come up there has to be an improvement in their funding, faculty, administration, and marketing, so that they can pull a lot more Black students. At the same time there also needs to be better K-12 education for Black students to increase the college-prep level of the kids, both to increase enrollment at HCBUs and to ensure that more of the students who enroll there are actually ready for a college-level curriculum.