I dont think a super league of 20 teams is viable. I also dont think one of 72 teams is either.
If there is ever a super league that breaks away it will almost surely have to be somewhere in the middle. More in the lines of 36-48 teams, give or take. The money people will not want the lowest value teams, but the system will need enough teams to provide interest, content, and an overall system that works.
And the only way it happens, is if the people with the money want it to happen. If those people pull the strings, it will happen, regardless of the B1G and SEC wants. If their money brokers force their hand they will have no choice. But if those money brokers want to remain in the current system with conferences etc. It will not happen.