Until "P5" teams are required to admit any team that applies to join, there has to be an automatic means for teams in lesser conferences to control their own destiny and gain access without requiring the votes of someone else.
Win your conference - you are in the playoffs. Simple as that. Everyone has access, everyone has a shot.
Similar to basketball - bigger conferences will dominate the at-large berths. I can live with that as long as every conference is represented. Heck - I don't even care if you make G5 conference champs play each other to decide who gets in and call that the 1st round of playoffs. However it's done - everyone has a shot.