Your last sentence is very true. The only thing keeping the Big 12 afloat is that Okie keeps finding a way to be relevant in the NC picture. If that stops...
15 years ago, Texas and Oklahoma had a stranglehold on the Big XII South. But then A&M jumped ship and it all started falling apart for UT. Suddenly, the Aggies got good - Heisman good. So did Baylor. And TCU started showing up on the national stage. Now, I'd argue that A&M is consistently the top team in the state, while the Big 12 Texases play musical chairs trying to out-do each other.
If not for Oklahoma continuing to stay elite (Lincoln Riley is looking like a GREAT hire, btw) after Stoops' departure, the Big XII would have already gone the way of the Big East and become a basketball conference. Aside from a few flash in the pan years, ISU, KU, KSU and OSU are really non-factors in the football landscape.