There also may be a fundamental philosophical difference between the B1G and SEC. The B1G may still want there to be only 2 conferences to rule the CF world -- them and the SEC. The SEC I think recognizes for the good of the game there needs to be more than just the two conferences. (Among other differences.)
As you note, the SEC has identified that a power league of just the B1G and SEC is not likely to succeed. Also, the SEC remains a regional conference, tightly packed into the Atlantic south, Gulf Coast and a few more interior teams, all geographically packed into a small region.
Add to the above the other issues, such as:
- ESPN will not kill off money makers to please the B1G, they may expand elsewhere to maximize profit but not slice their throat, the ACC is safe for now
-Hoops is a money maker, 25% of conference media deals, you cannot destroy sources of income when you try to maximize income.
-Fox and ESPN make money off the Big12, again, not slashing their own throats
- If too many schools are left out, CFB viewership will wane, fans watch their team first and foremost then watch other games.
-Not many people care about Illinois v Washington, Mississippi v. Mizzou, etc.
-Many more metrics that TV analysts use to make decisions that this site has not discovered.