I'll go ahead and take a crack at this. If I had my way, I'd go back to the BCS. I prefer a 2 team playoff verses 4, but I can live with the 4 team format we have today. Going to 8 would be awful.
The college football season is very short. It only lasts 3 months and its basically a single elimination tournament. Over the past 50 years or so there's only been two seasons where more than two major (P5) teams finished undefeated - 2004 (USC, Oklahoma & Auburn) and this year. Most years only one team gets through unscathed.
The fact that winning the national title practically demands perfection produces the greatest regular season in all of sports. Every single game is a must win. This creates an amazing environment every Saturday in the fall. Its why the anticipation for big games is palpable. Its why losing that game is devastating and winning that game is euphoric. Its also why fans excitedly switch over to watch whatever top team is struggling in hopes that they'll get upset, as any loss can't be a death nail.
One loss alone doesn't automatically eliminate a team, but it does mean your going to either need some help or get lucky.
All that excitement, angst, intensity and passion would dramatically decline by going to 8 teams.
Number 8 this year lost 3 games, one in blowout fashion and another to a team that went 6-6. Number 7 played an incredibly weak schedule but still lost twice to the only good team they played. Number 6 also played a weak schedule, lost their only decent non-conference game and then lost to a 7-5 team. Number 5 got blown out when everything was on the line and also lost to a 4-8 team at home. All of these teams had their shots and failed. There's no reason to give them another shot.
This years teams ranked 5 - 8 aren't unusual. Every year the 5th - 8th ranked teams failed at some point during the season. Those teams shouldn't be rewarded, but more importantly, the teams that already beat them shouldn't be forced to do it again.
The games played in Sep, Oct and Nov matter. They shouldn't be dismissed just so casual fans that don't pay attention all year long can have an extended playoff.