Here's the argument I have against the 8 team playoff. One of the absolute best things about college football is how every single week can potentially be an elimination game. All three months have huge stakes and huge, impactful games because of it. With an 8 team playoff, you're inviting teams with 3 losses to have a chance at winning the national championship, which seems lame as hell to me. The only time we've ever even a two loss team in the championship game was the most chaotic season in modern history, now we want to make the barrier for entry that loose? I don't really like it.
The only two ways I would be cool with an 8 team playoff are as follows:
1. P5 conference championship games are the first round, with the conference that has the lowest aggregate ranking of the participating teams (so this year it would be the Pac-12) being excluded for the year (this is a bad version, obviously)
2. All 5 P5 conference champions are automatically in, with 3 at large teams, but any undefeated and/or top 15 ranked G5 teams are automatically in, and and nobody with more than 2 losses can qualify unless there aren't any other teams to pick from.