If the remaining 8 schools only add AAC schools and not BYU, it would send a 100% very loud and clear message that they have no interest in trying to field the best and strongest possible schools/competition for the Big 12 moving forward.
BYU already has better facilities than most of the remaining 8 schools not to mention the AAC schools. BYU has the largest football stadium of the group. BYU Athletic Department already competes on the level of a P5 school. BYU brings more cachet than any other possible expansion target.
BYU with a P5 moniker attached to athletics, and BYU would quickly turn into a powerhouse.