1. The ACC will pay out more money than the Big 12 over the next decade, even without their top teams because ESPN can't rework the contract just because the composition of the conference changes -- as long as the conference remains above 15 teams. The ACC network will continue to grow revenue as it expands westward thanks to SMU, Stanford, and Cal, plus when the Big 12 contract expires they will face the same stark reality the Pac 12 faced - they are not worth the money they think they should get. So the Big 12's next contract will not be all that meaningful.
2. Also, the GOR will not "open up". The addition of three teams actually made this even more certain - there will not be 10 votes to disband the GOR and no court is going to throw it out. Thus, the few schools leaving will be paying a ton of money to leave. Louisville, Va Tech, Pitt, etc will not be able to afford the exit fees ... and they would have to give up their share of the money being paid by FSU, Clemson, etc -- which could be more than $1 billion in total. By the time the cost to leave gets more reasonable in the 2030s, the Big 12 will have a new contract that will be on par with the ACC so even in 2032 or 2033 there won't be any real incentive to leave.
You can forget the Big 12 getting any current ACC team.