Example of a very poorly-designed postseason:
12-5 Rams
@
8-9 Panthers
.
Yes, the NFL is the #1 sport in our country. Yes, everyone knew the rules going in.
That doesn't mean it's a sane system.
It's actively bad.
Hopefully college football doesn't do the same (despite last year's top 4 seeds...)
Yet the 6+2 model that you prescribed (3-team superconferences of 20 teams each, broken into 2 divisions, every division winner gets an automatic berth) would certainly end up with teams who are the equivalent of those 8-9 Panthers.
Yeah, those teams would probably be no worse than say 9-3 or 8-4, but in the non-parity world of CFB relative to the NFL where teams play a lot of patsies, 9-3 in CFB is basically the equivalent of being about a 0.500 team in the NFL.
In 2000, Purdue won the B1G with an 8-3, 6-2 record. They won it by virtue of a 3-way tie with Michigan and Northwestern (who they defeated H2H). They had two bad losses (PSU and MSU, who both finished with a losing record. Yes, that wasn't a full round-robin schedule in 2000 as it was 8 conference games and an 11-team conference, but the teams they missed were Iowa and Illinois who both finished with losing records. They were ranked in the top 10 for only one week all season, at #9.
Had the 2000 Boilermakers been in your system, I'm sure it would have generated some grumbling about an "unworthy" CFP team considering this was the final pre-bowl poll:

Purdue was 13th in the AP, 14th in the Coaches, and outside the top 15 in the final BCS standings. If we assume the old conference system the other 5 conference champs were Oklahoma (B12), Miami (bEast), FSU (ACC), Florida (SEC), and Washington (P10).
You're telling me that you don't think there'd be grumbling about "unworthy" 8-3 Purdue being given a CFP berth instead of making enough room for teams like 10-1 Oregon State, 10-1 VT, 9-2 Nebraska, 9-2 Notre Dame (H2H win over Purdue), 9-2 Oregon, 9-2 Texas?
I don't buy it.