The Big 12 is theoretically salvagable. If Texas wanted to stay, the Big 12 could easily expand and survive. But it's nearly a certainty that Texas will leave, and the conference will continue taking on water until Texas makes a decision. The remnant could, if they had the will, choose to add teams. But by then it will be so weak that schools are going to be a lot more concerned about getting a good conference for their own school than in whether or not the Big 12 survives.
The Big 12 has only been around since the 90's, it's not like it has deep roots.