Texas A&M blinked.
There's really no other way to say it. A&M didn't want to be seen as the bad guys after Texas maneuvered the situation so that the survival of the Big Roman Numeral depended on the Aggies. When the conference was being killed by another's hand, the Aggies had no problem ditching their former conference mates, but they couldn't bring themselves to be the trigger man. In a way, that's admirable. The Aggies are who we thought they were. They are a loyal breed, and even though killing the big Roman Numeral at this point would be a mercy killing, A&M couldn't be the ones to do it.
They aren't killers. Texas, on the other hand, consistently showed that they only cared about two things: money and blame shifting. Give credit to UT, as the ground shifted beneath them, they showed a remarkable ability to adapt to the changing situation and still use it to their advantage. By Monday, they had found a way to force A&M to decide on the fate of the conference. If A&M went to the SEC, Texas got to go to the Pac-10, a destination they have long since desired. If A&M stayed with the pack, Texas could portray themselves "savior of the conference" while also remaking the big Roman Numeral into an even more imbalanced conference.
The Big XII started to fall apart due to the institutional factors leading to a competitive imbalance. In order to "save" the conference, Texas actually made the competitive imbalance worse. Texas also got a bag of cash from FOX, who saw an opportunity to get into the college football game again. Texas gets more money, the ability to set up their own TV network, an even larger revenue imbalance among the conference members, got rid of the conference title game which the coaches hated, AND jettisoned one of their toughest on-field competitors. The Horns are who we though they were. They might be sort of evil, but they played this whole scenario out with Machiavellian brilliance. More money and an easier conference? Sign me up.
The open question is how long this can last. The Big Roman Numeral is essentially a sham conference. A&M's power play allowed them to get a seat at the big kid's table, but this conference is clearly set up to be Texas, Oklahoma, A&M, and the Seven Dwarves. Baylor and Iowa State are only in this conference to cash checks and lose to the two power schools. Mizzou and Kansas are allowed to stay just for basketball and their TV markets. But it is clear that Texas does not view any other school, not even Oklahoma, as an equal partner. This is the Texas Conference, and the other schools are expected to field decent football teams that will dutifully lose to the Longhorns each year.
Frankly, this conference cannot last. The member schools are only still members because of either fear or borderline bribery. Kansas found out exactly how much the other conferences value their vaunted basketball program (not much). They have almost no leverage. Mizzou went from trying to attract the Big Ten to now being thankful they still have a conference. Baylor... well, Baylor never really had any illusions about their good fortune to just be along for the ride. Texas Tech and Oklahoma State found out exactly how much their recent success has mattered to the bigwigs (almost none). This is a conference held together with duct tape and bailing wire.
What this deal does is that it buys Texas time to build the Longhorn Sports Network. That way, when they make their next play to join the Pac-10, the conference will be forced to accept the LSN as a fait accompli. In five or ten years, when Texas tries to make this move again, it is an open question whether they will bother to throw anyone a life boat again.
Texas A&M saved the Big Roman Numeral. They also forced Texas to, at least temporarily, treat them as an equal. Now, they are forced to answer the question: was it really worth saving? And has A&M really done anything other than just make Texas more powerful?