It's not THAT they're leaving, it's HOW they're leaving. Just like it's not THAT Nebraska left, it's HOW they left.
A&M is one of the 4 schools (along with Nebraska) that for more than a decade voted for revenue inequity in an attempt not only to separate the haves from the have nots, but to become revenue king of the conference, therefore becoming the one holding all the power. Just like Nebraska, they didn't count on sucking in football for the better part of a decade, which eroded at their money and power. Meanwhile, Texas became revenue and power king of the conference, and instead of looking in the mirror to see what they did wrong, they found it easier to point the blame finger at Texas for their failure at becoming the school in conference to have the most money and power. And instead of trying to stick it to Texas by voting with the rest of the conference to share revenue equally, they decide to try to stick it to Texas by destroying the conference, be damned whether the smaller schools in the conference get hurt in the process - because it's not truly about how Texas is treating everyone in the conference unfairly, it's that A&M isn't the one that gets to treat the conference unfairly.
You want to know why it's not just about THAT A&M is leaving? Ask yourself how many fans are truly upset that Colorado is gone - not very many, and if so not to the extent of Nebraska and A&M. Colorado, while wanting to go to the Pac 10 since the old Big 8 days, wasn't one of the ****s when it came to revenue sharing and power, and left because they thought they saw the writing on the wall, not to mention they got the offer not only could they not refuse, but have wanted for decades.
Nebraska and A&M, on the other hand, have been power hungry since day 0, and when they realized they lost the arms race to Texas they decided to try to stick it to Texas by destroying the conference around them, regardless of how many ADs they destroy in the process. That's why they don't get sympathy.
And yes, if ISU were to get a Big 10 invite, ISU would probably be gone the next day - assuming the Big 12 fails in landing ND/BYU/Pitt...