1.) They would be locked into a long GORs and a low TV payout.
2.) FSU, Clemson, and Miami will jump for the SEC if the chance comes.
3.) UNC, UVA, and Duke would jump at the chance for a B1G invite.
4.) If ND jumps to the B1G, the value of ACC home games in the current TV deal drops (losing a huge draw).

At the end of the day, there is NO guarantee the ACC will still be what it is. Just like the PAC 12 there is no certainty or assurance it will be viable. At least with the Big 12 you know all its schools are behind the top PAC and ACC brands on the pecking order of B1G and SEC expansion targets. No Big 12 school is leaving a somewhat stable, though not ideal, situation, for an uncertain and unpredictable one.