According to ESPN, the ACC has just signed a media rights deal through 2026 that secures the media rights of members with the conference until then. Meaning that if a school were to defect to the B1G, the media rights to said school's sporting events and home games would still belong to the ACC, deflating any incentives for the school to defect. The B1G would not profit from additions from the ACC outside of games played at current B1G schools.
I'm not familiar with the legality surrounding such a deal, but assuming it holds up until 2026/27, it could prove to be a major roadblock to conference realignment. I dont see many schools outside the ACC as being worthy candidates of B1G addition. The ACC has effectively poached the top choices from the big east, so unless the B1G goes after Cincy or UCONN, its expansion could end with Rutgers and UMD.
Personally I'm desperately hoping there is a way around this because I really want to see the ACC fall at this point, and had high hopes of taking UVA and some others with us.