The power conferences already have all of the real power, thinking they don't is silly.
There is an additional benefit to being in a conference governed by the NCAA that would disappear if they split from the NCAA, exclusivity.
Where were UCF and USF 65 years ago? If we had the resources, you, I and a few boneyarders could buy some buildings, hire professors and administrators and over a number of years build a credible university. At some point, after meeting whatever criteria the NCAA has to become a member, we could apply for membership and they would have to accept us. Groups within the organization can be exclusive, the organization itself cannot.
If the B1G and SEC were to leave and start their own organization (the be a part of instead of the NCAA) and some school that didn't get to go along applies, what happens if they are denied? I imagine a lawsuit that the new organization knows going in they cannot win. The new organization won't need to give a spot in the B1G or SEC, but they do need to admit them in the organization (if the criteria is met, and, if the criteria is unreasonable, that will fall under legal scrutiny as well).
It won't take long for 45% of D1 to apply for admission.