First, I reject your premise that G5s could shut out BYU. G5s wont be making the decision on who is included in an 8 team playoff. I think it unlikely in any scenario that BYU would be shut out from consideration. It would be something like "highest ranked non-P5 team."

But I do think playoff expansion is almost inevitable and 8 teams makes the most sense. The model that the tweet SAYS is being discussed includes one G5 auto slot (if true, would that force BYU back to a G5?). I think that unlikely. If you are P5, why should you guarantee even 1 spot for a G5 team, especially when you have expanded the number of teams? All you have to be is ranked top 8.

But even without a G5 auto bid, what does that mean for BYU? I think the expansion would mean that in some years you would get a G5 in the playoff. The occasional undefeated G5 by most metrics would rank in the top 8. But what should BYU do? I think they would then need to schedule more strategically because you do have a shot at the playoffs. Avoid CFB elite teams like LSU, etc., and BYU might have the ability to perhaps game the ranking metrics because of the inability of G teams to schedule many good games. More mid/lower P5s and the top of the G5.