The question was about stability and longevity. So let's consider the facts:
-Within the last 3 or so years the Big 12 lost
4 schools. When those defections happened, there was a lot of talk about Texas and OU flirting with other conferences. The general consensus was, and still is, that if Texas and OU leave the Big 12 conference will fold. The conferences they talked to were not willing to allow Texas to keep the same arrangement they currently have with the Longhorn network, and as a result Texas stayed in the Big 12. OU followed suit. They signed a GoR and the Big 12 is stable for the time being. In order to partially fill the void created by those 4 teams leaving, the Big 12 was forced to reach halfway across the country and take WVU.
-The ACC is about to lose Maryland. This is the first school to leave the ACC in 42 years. This was the result of their old AD basically bankrupting their athletic dept. Their new AD, who has no ties to Maryland or the ACC, was forced to make a hard decision because so many sports were being cut due to the previous AD's incompetence. It is a pretty unique situation among ACC schools. After this news was announced, there was a lot of talk about the ACC getting raided. But most of that chatter seemed to originate from blogs and sports boards (I think the traffic on WVU boards probably tripled during this time.) The ACC presidents unanimously approved a GoR, and the conference is stable for the time being. In order to fill the void left by Maryland, the ACC grabbed Louisville. While they are not located in the current ACC footprint, they are at least in a state (KY) that borders a state (VA) with two current ACC teams.
-The ACC is working with ESPN to launch an ACC-ESPN network. Due to the location of ESPNU studios in Charlotte, NC the start-up costs for this network are going to be far less than those for the other major conference networks. The ESPNU studios and broadcast facilities will be a big help in the infancy of this new network. The ACC's footprint covers more television viewers than any other conference. ESPN has proven successful at shoving new networks on cable/satellite providers over the years.
-The Big 12 has no plans to launch a network of their own. They can't because all of Texas's tier 3 rights are tied up in the Longhorn Network. There is no market for a Big 12 network that doesn't feature Texas. Even if they somehow convinced Texas to give up the Longhorn Network, the footprint of the Big 12 as it currently stands is rather small in terms of size and television sets. So of the 5 so-called "major conferences" the Big 12 is the only one that won't have their own conference network within the next few years.
-The ACC lost Maryland and replaced it with Louisville. From an athletics standpoint, Louisville is an upgrade in most sports over Maryland. Their athletic department is clearly in better financial shape than Maryland's. Their facilities are better than Maryland's facilities too. The one area in which losing Maryland hurts the ACC is the loss of the DC television market.
-The Big 12 lost Texas A&M, Missouri, Nebraska, and Colorado. They replaced those 4 schools with WVU and TCU. I'm not sure there is anything about that trade-off that can be considered a positive for the Big 12.
-The Big 12's current GoR is set to expire about 2 years before the ACC's GoR does. Meaning that the Big 12 will have to once again deal with the question of whether or not Texas and OU will leave and destroy the conference, while the ACC is still bound together by their GoR.
You can draw your own conclusions from those facts. But a couple years ago the Big 12 was very close to falling apart. The ACC has never been in that situation, except in the minds of a few WVU fans with very big imaginations.