SEC is definitely their priority property but the ACC also makes them a ton of money. I don't like ESPN, but the fact is they gambled with the deal and won. At the time they made the deal, they offered the ACC more than what other conferences were getting because they expected college football to continue to grow. The risk was that the conference could be garbage in 10 years, with almost no viewers. If the worst came to pass, they would still be paying the 40-50 million per acc team, even if losing money. The bet paid off for them.
Other conferences built in shorter windows so they could re-negotiate, but the trade off is they take on more risk of having some bad years, and the next time they don't make as much money (or the major networks have made deals with more lucrative conferences and aren't interested in the price the conference is asking). It's like a 22 year old NFL player signing a 3 year deal worth 30 million instead of a 5 year deal worth 45 million. He's gambling he's going to have 3 great years and be in line for a megacontract when he's 25, but trading the chance of having guaranteed income.
SEC obviously isn't going to add all the ACC teams to the SEC, so they would probably add 2 teams. That means 12 teams would go to other conferences and they would lose out on the revenue (or at best a fraction of what they got if ESPN has revenue sharing with other networks). The amount of money ESPN would make by shifting 2 top market teams to the SEC isn't going to make up for the amount of money lost by the other 12 teams going to other networks. They aren't going to let the ACC dissolve to try to get out of the contract. Simply not how the real world works.