It's all silly and stupid, but ND isn't going anywhere until they have to. If they get a $60-70 million a year deal from NBC, they will be an independent for as long as that deal runs. And, if they're forced to go somewhere, it won't be to a sinking ship like the ACC. So, the ACC would have potentially added dead weight in Cal/Stanford - for what? An opportunity to be rejected by ND?
Now, one actually plausible scenario - IF the ACC implodes and is picked apart by the other three, ND will need a home for all of its non-football sports if it remains an independent in football. The Big Ten and SEC will tell them to suck eggs if ND asks to be a member of those leagues in all sports except football. And that leaves the Big 12 in pretty good shape as they would absolutely take that situation.