The Big 12 could survive, but it would be very tough to survive as a Division 1 conference. Here is the most talked about timeline for the future of college football:
1) Nebraska's done-and-waiting-official-release deal with the B10 would cause the Pac 10 to make a move on Colorado, and then eventually invite 5 other schools from the Big 12 (mainly the South schools).
2) With the new Pac 16, the Big 10 needs to make a move to make themselves into a larger conference. As such, they invite Missouri, two teams from the east, and Notre Dame. Notre Dame, not wanting to get left behind in the super-conference dust (which, if they don't join, they will be sorry) will join the B10 thus creating the Big 10 +6.
3) The SEC will make a move to grab teams (like Miami, FSU, VaTech, etc.) forming their own super-conference, and then the ACC, Big East, and a few other schools will probably create a fourth super-conference.
In the end, the B12 is left with no key-note team. The second Nebraska leaves the B12 North, the North loses pretty much all of the respect and tradition it had. There would be no team in the nation that could replace the void Nebraska leaves. Similarly, no team could replace Texas. The Big 12 will either grab up the schools that couldn't get into the B10, SEC, or Pac 10, or they will drop all of the schools in the conference into DII.