The fact that we have subjective rankings and the SEC method means there are going to be faults in the ranking system. Moving to 8 teams will have an undeserving team or two included every year, this is true, but they wont get 1 chance to luck their way to a championship, they will have to do it 3 times. A team that can win most/all of their regular season games and then go beat 3 top 10 teams is your best team.
Moving to 8 makes it more likely the best team is included. There's a serious lack of cross conference play to determine who is the best without including a large enough sample size in playoffs. With 5 P5 and a G5 with some good teams, domer, and times when the best team had an off day like Bama at barn last year, there's often going to be a case for more than 4 yet almost never a case for more than 8.
Its possible to have 6 undefeated P5 including ND, and a G5 often goes undefeated. Why would you not have a system that has room for those 7 teams?