I think the Big 12 will be fine long term. Yes, the SEC and Big 10 will be the premier conferences and will be overrepresented in the playoffs. But the Big 12 will survive because the fanbases care enough to make them survive. And because they don't have any remaining blue bloods, they will have stability while the ACC is on constant poach alert.
Last year's Big 12 Title Game drew almost as many viewers as the Big 10 and SEC title games - and didn't feature UT or OU. Once the PAC 12 is gone, they will get a share a solid share of the corner school eyeballs as well.
It is hard to predict who will be the dominant Big 12 team over the next 20 years. Colorado looks like a good choice, but who knows how long Prime stays there. Utah has had success in the Pac 10. It could be them. Tech seems committed to the NIL game and seems better positioned than the smaller private schools over the long term. And Kansas is on an up-swing and has the added stability of their basketball program. Honestly, if their football has a decade of success, they would be prime to get poached by the Big 10.
It will be interesting to watch. In the meantime, I hope we mud hole every one of them on our F you tour.