The most likely scenario for Notre Dame joining the ACC in football, IMO:
1. The Irish go many years without a playoff berth, while the ACC champ keeps making it
2. The ACC beats the Irish on a regular basis, making it look less like a step down
3. NBC signs up a conference like the AAC for football, making ND less important
4. ACC media rights continue to increase in value.

All of these are plausible, and together put ND in a "why not join" situation. The ACC will not attempt to "force" ND to join for football, however.

JMO