for an individual conference member.

In a vacuum, sure, adding Utah would benefit the Big12 more than not adding anyone at all. But there isn't a vacuum, and there are many schools who would benefit the Big12 MORE than Utah, and would be just as likely to seek an invitation if any schools started abandoning the Pac.

And even assuming it did end up being Utah, and their presence strengthened the Big12, Utah being on equal footing with BYU lowers BYU's ceiling. Yeah, it sucks that it's in any way a zero-sum game between BYU and Utah, but that's the case. The two schools compete for recruits, fans (yes, fans. There didn't used to be a kid south of point of the mountain who rooted for the U, and not many in the southern half of Salt Lake Valley either. All that's changed, but given a decade and shift in fortunes, it could easily change back), business sponsorship, and media coverage. One school's gain is the other's loss.

Therefore, yes, the Big12 having both schools is probably good for the Big12, but a strong BYU and a G6 Utah is MUCH stronger than two schools who keep each other from being as strong bringing from the conference being slightly better positioned all told.