I thought the playoff has supplanted the BCS. The P5 designation can remain with a weakened B12, the only real meaning it has is voting rights on NCAA football matters and automatic berth(s) in New Year's Day bowls that are not part of the playoff that year. The automatic bowl berth(s) may get yanked if the 2 big dogs leave the B12 in the future.
you are correct. I'm calling the new format BCS although technically it is the College Football Playoff.
If OU and UT are out of the conference before that contract expires (originally 12 years I think), the Big12 as a P5 conference is dead. It's the automatic berth in the NY6 and voting that matter and they would certainly lose that. The Big12 would be relegated to G5 status almost immediately under the new contract unless there is a complete overhaul in the structure that doesn't resemble today's build.
If the new Playoff contract is signed while UT and OU are members, I can't see the Big12 getting left out or anything materially changing. In that case, the Big12 may have bought enough time to sustain itself by grabbing some additional schools and allowing them to develop over time. However, when has anything like that happened? It doesn't because nothing happens by accident or by chance. Everything that is done creates more money for fewer mouths. So with that, what you might see is a change in the wording or some change definition change of what is P5 allowing for conferences to be evaluated after the departure of more than 1 team or some other metric (essentially allowing for the Big12 to be reclassified as a G5 should UT and OU leave)... I think you might also start to see the term P4 mentioned more and more...
From that standpoint, I think the Big12 will be very much like the Big East. It's value will be constantly challenged and questioned until perception is reality. Big12 and ACC knew what the stakes were. The Big12 is now the weakest conference, not by opinion but by fact. Only a matter of time before schools of value jump to one of the other P5 conferences (OU and UT are guaranteed a spot along with whoever they take with them). The others will get the run-out on the P5 contract (maybe elevate a couple programs in that timespan (Cincy, UofL and USF), and then go the way of the AAC and MWC.