My uncle is a prominent USC booster and has mentioned several times to me that the Sunday rule was a major reason why BYU was not added to the P12 last conference realignment. He wasn’t in any room when decisions are made, but he is very well connected.
Sunday play was not the only reason, but a big one. Taking away Sunday play impacts already hectic schedules for the non-revenue sports and that’s a big head ache and cost.
There may be some scenario where the B12 is so desperate that they add BYU, but I highly doubt it.
I would love to see BYU in the B12 or some other P5 conference, but don’t see it happening.