I like making conference championships a requisite for playoff entry, but there'd be way too much heartburn if there aren't at least a couple at-large spots. People would point to Ohio State this year or some hypothetical fluke where Bama goes 12-0 and loses the SEC champ game. You or I may place value on championships, but we're seeing this year that the powers-that-be don't necessarily agree.
If it went to 6 with the 5 P5 champs, you could pretty much guarantee the 6th spot would not be held for a G5. I think you have to go to 8 to have a reasonable chance of getting a guaranteed slot for the G5.
I really like the 8-team model, with 5 P5 champs, highest G5 champ, and 2 at-large. It makes winning a conference championship a priority, it gives the G5 access, and it allows for 2 great teams who maybe blew a game to still get in. Best of all, it (almost) guarantees that an undefeated season by any team will give them a shot.