agree to bring them on board NOW, there's no guarantee that they will in 5 years either. They increase their chances of a Big Ten invite being in a viable "Power" Conference. The Pac 12, in its current state, is every bit as "Power 5" as the Big 12, but if they lose a couple of teams, even CU/UA, and replace them with schools clearly below the P5/G5 cutoff, they lose that viability.
I know some on CB would disagree with the sentiment but a Pac 12 with Oregon and Washington is as good as a Big 12 with 3 G5 adds and Independent BYU and no blue bloods.