ESPN isn't going to carve up the ACC until their GOR expires. Why put all of their investment in the ACCN only to tear it apart?
Most of the ACC won't agree to go to the Big 12 and kiss Texas's shoes unless they are out of options. At least half of the ACC would have to leave for the Big Ten and SEC and if the right teams (Clemson, Florida State) are left out, they can reload and insist on making ESPN honor their GOR rather than go to the Big 12. And the plan to disband the ACC would require the Big Ten to take some ACC teams and unless ESPN gets all of their 1st tier football and men's basketball rights, they lose their rights. Now when football's involved the most likely teams the ACC will lose (UVa, UNC) have limited football value but they'd have pretty big men's basketball value. ESPN can either keep all of the ACC's value or give up some up it.