There is no way an ACC school is leaving the ACC with their current GOR and media contract, they would lose way too much money for 15 years.
ACC schools are not going to be available for the B1G, its going to be if and what Big 12 schools are the other conferences going to take, because they will be going somewhere, so take them or they will go somewhere else.
What is happening is you have 10 schools that are being thrown into a bucket, with 4 conferences vying for them, 2 schools have been hooked by one of those conferences, leaving 8 schools, for the other 3 conferences to decide if they are going to keep up with the big kid on the block, to take or let another conference take them and possibly leave you behind.
You have Power 5 caliber programs available right now for expansion...where otherwise you only have G5 schools to take if you expand, if you sleep on this other conferences will take them and you will lose out. So I dont see the 3 other conferences not doing something to try to scoop up the rest or most of the rest of the Big 12 before some other conference makes a play for someone they might want.
An Iowa State, Kansas IS way more enticing for expansion than a Cincy or Memphis, and if the B1G misses the chance at us and we go to one of the other conferences and a couple years they decide they have to grow to match the other conferences it will be too late.
This is the issue right now the entire Big 12 is destabilized and the other conferences know it, and the SEC is making a play to jump to the front and control the power structure. The other conferences will have to do something to match them or be left behind.