Big 12 has to have 10 teams at the minimum for Fox to honor the 2nd tier rights. So they'll get back to 10. Texas and OU are openly against the championship game and 12 teams though. The championship game probably wouldn't be an overall moneymaker based on the value of any 3 teams added.
Also, IMO, something that isn't brought up is I think UT is still annoyed from 2007 or 2008 when them, OU, and TT all finished with the same Big 12 record but they were left out of the championship game. With no championship game, it brings back the possibility of shared champions. I think this is something that is appealing to the big schools in a backdoor way because they will likely have some happier fans as it likely means more championships.
As for long term stability the only thing that will give that is getting schools the best deal possible, at least comparable to the other conferences. Also, it's figuring out how the LHN will work within the conference. This whole thing is about money. If the Big 12 can find out a way to maximize their revenue relative to other conferences it will be fine. If not, in 6 years we're right back to where we started.