Actually - I will agree with your premise and agree to no G5 in the playoff - if
If we also agree that Florida State, Michigan State, Oklahoma, Ohio State, Alabama, and Notre Dame are also excluded from ever participating in the playoffs.
All of the teams listed above have lost a playoff game in blowout fashion - by 4 touchdowns or more. If we want to raise the bar just a little and also exclude teams that have lost by 3 touchdowns or more, you can add Oregon to the list of teams that never get to be in the playoffs.
Because, you know, we want to avoid blowouts. And as we now have a record showing that the above teams are all capable of being blown out in the playoffs, there's no reason to include them going forward.