If the plan is still to expand the CFP to 12 teams with the top 6 conference champions, then the Big 12 will still have plenty of incentive to stick around.

The concept of the P5 will be gone but that does not mean it will now be a P4. Instead there will be a clear pecking order now established in conference revenue

1. SEC (with Tex and Ok)
2. Big Ten
3. ACC, ND
4. PAC
5. Big 12 (with Hou, Cincy, Memphis, USF, UCF, BYU)
6. AAC
7. MWC
8. CUSA
9. SBC
10. MAC

So the Big Ten, ACC or PAC won't expand unless it adds value to their conference. Otherwise they will stand pat to hold their place in line. They will still get at least 1 team in the playoffs almost every year.

The only thing the Big Ten and ACC could do to keep up with the SEC is for the best teams to leave the Big Ten and ACC and form a new super conference. Maybe starting with PSU, Mich, OSU, Wisc, Clem, FSU, ND and adding 5 more. Of course I can't see those Big Ten schools doing that to their conference mates, so I think the Big Ten and ACC are stuck.