Because there are bigger forces in the world than Scholz and Micco. Just because either one of us says something's not possible doesn't necessarily make it so. There are some who believe the GOR isn't as iron clad as you might believe. We'll never know unless it gets tested in court, but I think Boren might be staging that battle.
The odds of the GOR coming apart without at least several of the schools in the conference (i.e., more than just Oklahoma... more than just Oklahoma and Texas... more than just Oklahoma, Texas, and OSU... probably some sort of contract-y type number like 2/3 or 3/4 of the schools) wishing it to be so isn't great. If Boren really wanted to leave, he'd either probably want to make it seem like it would be more trouble than it's worth to keep them from leaving somehow (which is difficult when the actual money cost is such a big factor) of sew dissent into as many other schools as possible (which I guess making a fuss over expansion might do, but I'm not sure that would make teams want to leave or blow it all up since so many of the schools don't have the cozy landing spots they do).
But even so, I think your original question is flawed in that, I'd imagine that most of the interested conferences would strongly prefer getting Texas and Oklahoma together (and possibly taking another team or two if that's what has to happen to make it so) rather than take just Oklahoma or OU and OSU as a set.
OU's best chance of getting past the AAU bar for the Big 10 would be some deal that also brought in Texas. And the Big 10 has said they don't really care about an upper cap on how many members they'd add, so, while I feel like 12 teams is ideal and 16 teams is a reasonable cap, their commissioner definitely made it sound like they'd consider 18 or 20 if they liked the situation. Adding Texas and taking OU along for the ride would probably be an ideal scenario for them. I feel like adding oSu would be a stretch though. I'm not saying impossible... but the Big 10 would probably like the sound of Kansas, OU, Texas and either Notre Dame (ideally) or Rice (a second team in Texas, plus academics) before they'd give much consideration to oSu.
And with the SEC, it'd be interesting to see how that played out. We have our Aggie friends who pop up over here to tell us that the SEC no longer has any interest in taking other teams from Texas. If that's true (and I think that claim is dubious at best... if Texas showed interest, I think the SEC would have to give it a lot of thought), I'm not sure how two Oklahoma schools would fare. Even so, they're far more likely to prefer adding two teams from two different states or, if they were adding two from the same state, have it be fore a specific purpose. For example, I think it'd be hard to disagree that the SEC's football product gets a ton of hype already and that, if they really wanted to get more money, they might consider adding teams that improve their basketball profile (since, when you already are making as much money as you can make with football, basketball is going to be the next biggest sport, and the number of games helps drive the value of a conference network and such). So, if they can't pry UNC and Duke out of the ACC, again, OU and Kansas... or OU and Texas seem like they'd be more attractive than OU and OSU.
And even with the Pac 12 option... same thing applies. The Pac would love to add Texas and OU, and if last time around were any indication, they'd actually be just fine with us bringing along OSU and someone else (Tech or whoever).
If either Texas or OU were to leave the Big 12 (either through legal maneuvers now, or more likely around the time the TV contracts come up for renewal), odds are that the other one wouldn't stick around. And from what I can tell, the same conferences would be interested in both schools... and would be interested in them more than the other options. So unless OU's prez (whether it's Boren, or whever came after him by the time this all happened) really just didn't want to cooperate with Texas... which is possible, I suppose...then I find it even harder to imagine a scenario where the two schools don't go to the next conference together than I find it to imagine the GOR coming apart before the contracts are up. And if OU were to decide that, in fact, they didn't want to cooperate with Texas and end up in the same conference, it might be far more likely that OU would be the one making the choice not to play Texas, if not in an actual decision, then in the way they make the actions.
If the two schools somehow go separate ways and it doesn't get crazy or messy? Sure, I'm sure they'd still play each other. But coming up with a way where the two schools go in different directions in the immediate future (particularly before the TV contracts are up) and it not being messy seems like one of the least likely scenarios discussed on the thread.