Expanding to 8 (conference champs in) puts an incredible amount of importance on the regular season. It takes away a TON of subjectivity. Nothing subjective about a team that goes through a whole conference season and emerges as the champ.Why assume that 5 through 8 wouldn't get boat raced? Recently history suggests that is the more likely scenario.
Additionally expanding the CFP waters down the importance of the regular season - every game is important. As it is now - 1-loss doesn't necessarily eliminate your chance to making the playoff. Now an 8-team playoff opens the chance that a 3-loss team could make the playoffs. Especially if conference champs are guaranteed a spot.