I'm not suggesting that ND will join in-full, nor that they would want to void their current contract. I was just answering your question about why the ACC would want ND as a full member.
As for the cable/carriage model, it may go away someday, but there is still money to be made for those already in the game. If the ACCN could miraculously drop out of the sky next season, it would still have a difficult and unpredictable struggle to get carriage. Having ND as a full member would cause a quick resolution.
BTW, I think ESPN's carriage contract with Comcast ends in 2025. I think ESPN will delay, delay, delay the ACCN until that time. If the cable/carriage model is still thriving, ESPN will, only then, consider the ACCN, in time for the carriage contract renewal, and the impending ACC media rights renewal. They have the ACC in their pocket until that time.