I think Texas realizes they would be a little fish in that big pond, and that's not palatable to them. They would certainly make a lot of money in the SEC, but they will NOT be able to bully around the rest of the league. Florida, Alabama, Auburn, LSU, Georgia - these are not schools that will be intimidated by Texas like Iowa State, Kansas, Kansas State, Missouri, Colorado and Oklahoma State were. They'll put Texas in their place fast if the Horns try to install a puppet Commissioner like they did with Beebe.
Texas has a lot more opportunity to call shots in a laid-back Pac-10, and when you combine the Seattle, Portland, San Francisco/Oakland, Los Angeles, San Diego, Pheonix and all the Texas markets together, you've got one heck of a footprint to use as a tool to negotiate your TV contracts. I think the Pac-Whatever conference with Texas would make a network as valuable as the Big Ten Network, and Texas still gets to call a lot of shots.
I think Texas to the Big 10, which will NOT let Texas run the show at all, is the least likely scenario.
I still think it's possible that the Big 12 adds a couple of replacement teams to cover for Nebraska and Colorado, and moves forward. But that's the least likely scenario, IMO.