Texas is leaving the Big 12 because the conference is full of passionate but unimportant teams, and the gap between Texas and the programs in its peer group is only going to widen as time goes on if Texas doesn't move. I don't even dislike the Big 12 that much, but program like ISU, KSU, OSU, Baylor, and Tech do not have the resources or the fanbases to draw sufficient national attention.
When you're good, people will say "neat" and immediately forget about it. They don't care that your coaches have great records or that you're "doing more with less" as developers of talent. No one gives a shit and they never will. Texas has nothing to win by beating teams like ISU and everything to lose.
The writing has been on the wall since Nebraska and A&M left, so bitch at them if you really need to bitch at someone. I would respect your fans more if you didn't, however. I'd rather you just said "thank you" for the years of unearned money and attention that you drew based on proximity to Texas and programs like it. Because Texas didn't kill the Big 12 - shitty programs with shitty fanbases on no resources did. Here's tip: if an employee leaves a company and the company falls apart, you don't blame the employee for killing the company. The employee was the one holding everything together. If you can't stand on your own then you have no one to blame but yourself, and certainly not the person who's been propping you up for almost 30 years.
None of this matters though. ISU (or OSU/KSU/Tech whoever) should learn to enjoy being king of a small mountain. You can still watch your team win and lose. You can still have your fun. The Big 12 isn't dying. It's just becoming what it has always been without teams like Texas - a second rate conference with a small audience.