The ACC revenues going forward are more variable than you think:
1) The ACC has benefitted from the uptake of the ACC Network, but they are at peak subscribers right now and subscribers will begin to decline as cord cutting continues, similar to ESPN. Is ACCN at peak revenue right now? (ACCN is ~12% of media revenues.)
2) NCAA basketball tournament credits. (Not a big delta, but Big 12 is earning more per team right now.)
3) Football playoff revenue distributions. Payout is changing. Last 3 playoff berths from future conference members: Big 12 two, ACC one.
As for picking off Big 12 schools, the timing wouldn't be ideal for the ACC as the Big 12 media contract is up 5 years before the ACC. If the market for college sports is good, the Big 12 will get a raise above the ACC. If the market isn't good, I don't think ESPN will let the ACC add schools. And, there are no Big 12 schools that will go to the P2, but a group of ACC may. Thus, the Big 12 will be more stable than the ACC.