So lets apply the above proposal to the 2019 season.
The Top-4 were:
LSU/Ohio State/Clemson/Oklahoma.
Teams 5-8 would have been
11-2 Georgia (who lost to LSU in the SEC Title game and unranked S. Carolina in the reg season).
11-2 Oregon (2 loss Pac-12 Champ with losses to unranked Ariz St and Auburn)
11-2 Baylor (who lost to OU in the Big-12 title game and the regular season).
10-3 Wisconsin (who lost to Ohio State in the Big-10 title game and the regular season).
(The highest ranked Group of 5 Champ would have been #17 Memphis at 12-1 with a loss to Temple. Keeping in mind Memphis lost by 2 TD's to Penn St in their Bowl which doesn't lend well to the idea of being competitive against the 8 teams listed above)
Not sure I see any of those next 4 teams having a season/resume's deserving of a shot at the National Title. So I think in 2019, the 4 team playoff worked well.
Lets Apply it to 2017.
The Top-4 were:
Clemson/Oklahoma/Alabama/Georgia
Teams 5-8 would have been
12-1 Wisconsin (Only loss was to Ohio State in Big10 championship game)
11-2 Ohio State (2 Loss Big10 Champ)
12-0 UCF undefeated G5 (Went on to beat Auburn which was Alabama's only loss)
10-3 USC (PAC-12 Champ)
OU was a Big 12 Champ - Lost to ISU (7-5)
Clemson ACC Champ - Lost to Syracuse (4-8)
Georgia SEC Champ - Lost to Auburn in Season, Beat Auburn in CCG.
Alabama NOT CHAMP - Lost to Auburn in Season, DID NOT PLAY IN CCG.
Wisconsin - Undefeated going into CCG with close lose to OSU
UCF Undefeated G5 - went on to beat Auburn who was the only team to beat Alabama.
Here Alabama had the luxury of not playing in a CCG and watching an undefeated Wisconsin stumble in their CCG. Clemson and Oklahoma were conference champs but had horrible losses. UCF had a good football program and never had a chance.
So what is important? Conference Champ? Not if Alabama is in, heck they didn't even play in the CCG... No bad losses, not if Clemson and OU are in.... Maybe its this subjective mix where people use the "eyeball test" and not hold all teams accountable to the same qualifiers to try to to determine an "unbiased" group of 4.
One could make so many arguments on who is let in....So by all accounts 2017, the 4 team playoffs were a complete dumpster fire.
The point being... We could sit here cherry pick all day. Maybe the expanding to 8 doesn't impact the final outcome in a lot seasons. However, when it does, it provides an injustice to the programs that were not let in. When you weigh the Pros and Cons of expansion its not even close...
At the end of the day you support a system in which it is possible for an Oklahoma State team to go undefeated, win the conference championship, and have the potential to not play for play for the national championship...Just because you feel that a 4 team playoff gets it right more often than not...