Why is the Big 12 important? Texas and OU. Take them out and you have a mid-major.
While Baylor has done well recently, TCU just left the MWC. West Virginia barely escaped the sinking Big East. Kansas is more sought after for basketball than football. Texas Tech, Ok State, Iowa State and Kansas State would likely become mid-majors if the Big 12 vanished.
Texas A&M bolted for the SEC with Missouri, while Colorado left for the PAC-12. The Big 12 is an unstable collection of mid-tier schools being elevated by Texas and OU. If either bolted, the conference would fall apart. If both left, it would probably be demoted.
The sad thing for BYU fans is that the Big 12's weaknesses are precisely when BYU might be get invited. Instability in a major conference is better than the instability of independence.