I actually think the B12 will fit nicely in the lower 3 P5. With OU and TX to the SEC, they clearly won't be at the top level of SEC and B1G, but they won't be G5. They have too much going for them, especially in non-football sports, for them to be at G5 level.
With that said, they will be as relevant in football as the ACC and PAC are.
I am definitely in agreement with your post at the present. However, on paper, the ACC and Pac12 should be a lot better than the Big12 eventually.
ACC is in an odd spot because they have teams that can draw eyeballs: Clemson, FSU, Miami, North Carolina, North Carolina State, Pittsburgh, Syracuse, Virginia Tech but it just hasn't transpired very well. Of the teams that I just listed, it is hard to find a Big12 team that actually has the tradition or fanbase to match any of these programs.
Pac12 is in same boat with programs such as Cal, Colorado, Oregon, UCLA, USC, and Washington.
I do think these leagues have their dead weights like all of the leagues (Washington State, Oregon State, Wake Forest, etc.).
I am not sure if these leagues are truly in decline or if this is just an aberration.
The ACC has always been the greatest headscratcher to me because I remember when they expanded in 2001-2002 timeframe, people expected the ACC to overtake all of the other CFB Conferences including the SEC (yes, this was a discussion point back then). You had the league that now had Florida State and Miami who were, at that time, two perennial super powers as well as had Virginia Tech (who was coming off the 1999 title game appearance), Boston College (who was regular ranked in the 1990s), Georgia Tech (who was more on par with UGA in the 1990s), Maryland (who had top 10 teams at the time), NC State, and Clemson (actually Clemson was seen as the weaker link back then).
I really think there was three coaching hires that perhaps changed the course of College Football. These hires are not paid much attention to but they are noteworthy:
1. Nick Saban to LSU
2. Mark Richt to Georgia
3. Tommy Tubberville to Auburn
In the 1990s, these three programs were not necessarily seen as national powers. The SEC was seen as the Florida-Tennessee show with sometimes Alabama making noise. These three hires really destroyed the FSU/Miami run (as well as killed Florida and Tennessee's run to a degree as well). The reason being, these three coaches started to take recruits away from these programs.
LSU emerged as a force in that era so the great players in Louisiana now started to go to LSU or SEC Schools instead of Miami, FSU, Oklahoma, Tennessee, etc.
Similar situation accrued with Georgia. Go back to Tennessee in the 1990s and look at the great players the Vols got out of Georgia. Mark Richt kept them in-house.
Auburn also started recruiting panhandle of Florida heavily with Tubberville hurting FSU.
This started the rise of the SEC and the decline of FSU and Miami.
