Even if they 'can' get out in 2031 for a fee it doesn't mean that 1) the fee still isn't prohibitive and 2) that they have another league to go to. If I were the Big 12, I'd be awfully careful about adding anybody else with the next TV deal, there's something to be said for having 16 members on the same page with realistic goals. Bringing in a Clemson or a Florida State has the potential to screw up that balance pretty quickly.
I don't think people understand how high the bar is now to be additive to the Big Ten or SEC. They both hit grand slams in the last cycle and they aren't going to settle for a single just because it's available.