First of all, nobody is leaving a P5 conference for the Big 12 right now. That's insane. The Big 12 is the weakest of the P5 conferences on the field and in terms of monetary value it's lagging far behind the SEC, B1G and even the ACC at the moment. Without a network in place you basically have a dysfunctional conference where UT is making money hand over fist and Oklahoma has one foot out the door.
Secondly (and this is more opinion than fact), in the unlikely event that the ACC loses 2 teams to the Big 12, you really think they are going to backfill? They clearly don't value us and if they lose two major football brands (FSU/Clemson) you think they are going to rush to replace them with two basketball brands (UConn and Cincy)? No way, my guess is they stay at 12(13) and keep their current TV deal for the next few years, get richer and then the remaining brand name schools bolt for greener pastures in the B1G, SEC or even Big 12 afterwards.