The reason to join a conference is not for playoff access.
It’s for bowl game access. It’s a rare occasion that BYU fields a playoff worthy team. Most seasons BYU will end up playing in a bowl.
If McMurphy is right and 4-10 bowl games get eliminated, this leaves BYU in a perilous spot. Bowl games in general prefer conference tie ins rather than to contract with individual teams. The P5s will be taken care of when it comes to bowl games. G5s will likely be able to retain 1-3 games per conference.
BYU doesn’t want to be in a playoff or bust situation. Joining a conference would give BYU greater bowl certainty most years.