Honestly it matters very little hypothetically how BYU would do in the PAC14 or Big12. In my opinion, it would be difficult to be competitive and if recruiting didn't improve BYU would probably be in a similar position as Utah is right now.
But lets not get ahead of ourselves here. Rejoicing in Utahs losses and more particularly their record in the PAC12 has nothing to do with hypotheticals, BYU's recent lack of success against them, the fact that there aren't any games between BYU and Utah for the next 2 seasons or anything else apart from them losing.
It's a great day for me when Utah loses. All the other stuff doesn't matter. If you don't live in Utah, you may not understand.