How much a team has or has not "proven" themselves means almost nothing if that success doesn't translate to revenue. Utah (the state) is a relatively small market, there's no way that the Big 12 will want to add another team just to try and get 100% of SLC (especially because Utah getting left out in the cold virtually guarantees that BYU will eventually own the entire state).
Ironically, I would guess that BYU would actually be the friendliest face in the room when it comes to Utah petitioning the Big 12 for entry, but I think more reasonable heads would prevail.