in CFB would be the G5. It would be easier for a P5 to make the playoff if it self demoted to G5.
Of course, no one will do that because the money and prestige is worse. So its not just BYU that is concerned about matters other than the playoff.
That said, I think BYU has irrational self confidence when it comes to its place in CFB. By perception, BYU is much closer to G5 status than P5. So in BYU's case, I think its kind of dumb to stay independent rather than G5 if the 8 team format is adopted.