We saw this play out last time around, when BYU dithered because of rumors that Texas and OU might leave for the Pac-10. Texas and OU ultimately didn't leave but BYU burned some bridges with Big XII members (who viewed BYU as a mercenary team rather than as a program that was willing to shore up the conference's reputation in its time of need). Notably, things turned out well for TCU, even though their P5 conference imploded shortly after TCU joined (TCU was a Big East team for a very brief time). TCU is now in the Big XII and a solid, respected P5 team.

We can speculate about superconferences and playoff structures and which teams will/won't make the cut. But I think it is self-evident that BYU's chances in any one of those scenarios would be better as a P5 team (a member of the shrinking in-group) than as a G5/non-P5 team (an outsider, trying to get into a shrinking in-group). 

After last season, I think a strong argument can also be made that BYU would be better-off in a number of G5 conferences, including as the AAC. Imagine what could have happened if BYU and Cincinnati had played in a conference championship game at the end of the 2020 season (instead of BYU having to play CCU on 2-3 days of prep just to keep up in the rankings)! IMO a G5 conference with the leftovers of the Big XII would be even stronger and more compelling than the AAC - so regardless of whether the Big XII maintains P5 status, BYU would be better off joining the Big XII.