It's not a playoff unless the rules for entry are determined before the season starts. There shouldn't be committees deciding, "Well, LSU didn't win their half of the conference, but they're really good," or "Well, Georgia lost in the SEC championship, but they deserve a mulligan."
If all the P5 champions are included, the P5 championships become playoff games. That means ten of the 65 teams in the P5 would already be in the playoffs.
None of the losers of the conference championship games should get in. Let the G5s & Independents compete for the other three slots. On the P5 championship weekends, the G5s could play each other and the highest ranked team that isn't already playing. That would leave a total of eight teams still in contention, and give the independents an opportunity.
It would create Cinderella stories, underdog stories, and most important, it would be a real playoff, where at the start of the season, you could get to the playoffs by winning.
It won't ever happen because the P5s will never let a system exist where a G5 could advance in the playoffs.