Once they see how much $$$$ the semifinal games bring in and start imagining how much more $$$$ quarterfinal games could generate, the ADs and presidents will be happy to do whatever they need to do to get out of the 12 year contract early and expand to 8 teams.
I think an 8 team playoff with 5 champs (SEC, ACC, B1G, Big12, Pac12), highest-ranked champ from another conference, and 2 at large teams would be a really exciting playoff. (And since 6 teams would be conference championships, you'd have several de facto Round of 16 games at the conference championship week).
That said, I do think what we're getting now is a nice compromise. Gets us to 4 teams - which in most seasons is enough to include any team with any reasonable claim on the title. Keeps the bowl system and the importance of the regular season (which I buy).