The point is that the opponent matters when it comes to media valuation. (You know that.) A G5 playoff opponent isn't going to draw like a P5, it will get even worse over time if the G5 participants don't consistently punch above their weight. Sure, as a playoff game, there will be more value than a regular season game, but the same rules apply. A 16-team, CFB committee selected playoff would guarantee better TV ratings and money than a 16-team, 10 CCG champs and 6-at large teams. Is any media partner going to risk paying out a large additional sum for the second over the first? Is there even enough value to consider it over an 8-team playoff? For better or worse, the system is venal. They aren't going to expand to 16 unless the payoff is worth it.
Also, a playoff doesn't have to include every group to be a playoff. It is just a series of contests to determine the winner of a championship. The selection process can be completely arbitrary or tightly regulated, the basic definition doesn't require one or the other.
I say the G5 are only in the same division out of convenience, so I don't see why that convenience sets up a threshold for 'deserving' full access to the championship. The rules for FBS inclusion are largely arbitrary and are set at some minimal number to not seem too restrictive, but not so low as to encourage every second tier college to jump up. It certainly wasn't devised on the concept of establishing a minimal quality of play. And I believe the rules were created pre-BCS, though perhaps they have tweaked them more recently.