To have ESPN as the common media partner is exactly why this scenario is a very real consideration.
Army and UConn already have their own non-ESPN media partners. They're not just going to step aside and let ESPN take over their inventory.
ESPN will do what is best for itself, and that could be AAC + BYU. That strengthens the AAC product (with 4 or 5 BYU @ AAC games) and ensures BYU inventory available for the 10pm ET timeslot.
*side note - this is another reason why I don't think it is a given that Notre Dame will run with an ACC-only schedule for 2020. ESPN and NBC will have a say in this. NBC's preference might be to keep its rights to the Notre Dame v. Arkansas matchup and ESPN may prefer to keep its rights in Notre Dame @ Navy game.