That's a terrible deal for the ACC and will guarantee that Virginia and UNC will immediately be moving to the Big Ten and others will be looking to the Big XII as well. The ACC brand will survive, but the remaining schools will be like Oregon State and Washington State and their ownership of the Pac-12. Florida State and Clemson leaving for free by winning their lawsuits is a big risk, but granting them an exit will guarantee that others will follow suit.
This is also a giant bluff by ESPN. ESPN isn't going to void the GOR - they know that the cost of live sports is only going to continue going up AND allowing the ACC out of the GOR early allows the ACC to better synchronize with the other power conferences for when their collective GORs all come up and would further increase the amount that ESPN has to pay. How much would FOX, CBS, or NBC be willing to pay for the ACC rights in a few years? Not as much as the Big Ten or the SEC, but significantly more than ESPN is currently paying and likely more than the Big XII would be able to offer as well.