I like to think of it in terms of "at what point do we want to have the argument about who gets included"?
With the BCS the argument was #2 vs #3. For a multitude of reasons that was far too fine a point. We were using a lot of flawed criteria to discern between teams that were essentially equal. Some team was always being treated unfairly.
It's an improvement to be arguing #4 vs #5, but as this year shows, I think it's still a bit too fine. At the end of the day TCU and Baylor were pretty much just as deserving as Ohio State.
I feel like arguing #8 vs #9 is a good equilibrium. At that point we're talking about 2 loss teams. Sure #9 (and probably #10, #11 and #12) would all be claiming how their wins are better than #8, and how their losses aren't as bad, etc. But... they lost two games. I think it's fair to let teams stub their toe once and still have a viable shot at the title. But I think it's also fair that in college football losing twice should most likely knock you out of the hunt.
True, if you have 6 or so zero or 1 loss teams like this year and last. But how often does that happen?
Zero or 1 loss teams versus 2 loss teams:
2002 - 4 vs 6
2003 - 3 vs 7
2004 - 8 vs 7 (2 of the 1 loss teams were in non P5 conferences)
2005 - 5 vs 6 (2 of the 1 loss teams were in non P5 conferences)
2006 - 6 vs 9 (2 of the 0 or 1 loss teams were in non P5 conferences)
2007 - 3 vs 9 (1 of the o loss teams was in a non P5 conference)
2008 - 9 vs 2
2009 - 6 vs 6 (3 of the 0-1 loss teams were in a non P5 conference)
2010 - 9 vs 6 (3 of the 0-1 loss teams were in a non P5 conference)
2011 - 6 vs 9 (2 of the 0-1 loss teams were in a non P5 conference)
2012 - 6 vs 8 (1 of the 1 loss teams was in a non P5 conference)
2013 - 6 vs 7 (3 of the 6 1-loss teams won their CCG, while 1 of the 2 loss teams won theirs against a 2-loss team that went to 3 losses)
2014 - 6 vs 2
(the perfect storm for an 8 team playoff)
(The notations above about non P5 conferences was added at what I consider the demarcation point of college realignment, the ACC's first raid on the Big East and then I stop mentioning non P5 conferences at the point in which their truly were now a P5 in name as well fact)
Now, if the trend continues of the past two years, I believe that will be a data set point that would be a plus for the power brokers themselves to eventually expand the playoffs to 8.
However, I believe they will search for data points that will give them reason to decide not to move to an 8-team playoff. And if I am correct in that belief, and what was the norm previously returns to be the norm then your point about "losing twice should knock you out of the hunt" becomes another reason not to expand to 8 - along with protecting the regular season, having the current CCG have more meaning than they would in an 8 team playoff, the question of whether or not a non P5 champion gets an auto bid, wanting the F4 to end by January 1 or 2, etc.
Again, time will tell.
Cheers,
Neil