Rutgers fans have openly professed a preference to the B!G but you are correct had the ACC offered them and there was no evidence the B!G wanted them, they would have jumped to the ACC without hesitation.
You are correct about regional rivalries being very important to develop fan bases. Currently there are no great football rivalries in the northeast. One could even argue there are no good football rivalries in the ACC, although the ACC has the greatest basketball rivalry.
Overall, you are correct about both. That said, I would offer up Florida State-Clemson as an example of a good ACC rivalry. Granted, its a level below the likes of Florida-Georgia, or, Texas-Oklahoma, but, both schools do have national recognition. Ga Tec-Va Tech has potential, if the Yellow Jackets can regain their form.
Cuse is geographically distant but BC is close and if UConn and BC could be strong in the respective sports and sustain that strength for a reasonable time it could generate huge fan interest. Unfortunately BC is more afraid of how UConn can impact them then they are positive of how UConn can help them.
Again, I used the Research Triangle Rivalries as a reference point. UNC-NCSU-Duke thrive as neighbors and rivals. BC-UConn could, too. I added Syracuse, because I think the three of you could form a tremendous northeastern leg of the ACC. If you all are inded No 16, I could see the league going to a pod system for scheduling. Just add Pitt to you three, and, let the fur fly.
UNC fans love to hate Duke but would never want to see the rivalry end. Same with Red Sox and Yankee fans. But BC's current leadership doesn't accept this paradigm for UConn and BC. Shortsighted view imo and as others postulated it already has impacted them athletically and could impact the ACC in the future.