it's membership is largely comprised of former C-USA members.
The CFP contract, which runs through 2025-26, recognizes the Big 12 as a P5 conference and nothing would happen if both OU an Texas left today. Their media rights contracts provides for pro rated increases and decreases based on the numbers of schools and not who those schools are.
After 2025-26, things would change dramatically (assuming OU and or TExas had left) and the Big 12 would become another American.