I really think if you make the top division any smaller than that, you start to weaken ratings.
That's why (well, and of course personal wishcasting) I don't think a Big 10/SEC break off would result in anyone making more money. If you start marginalizing wide segments of the sport and relegating them to a lower division, those fanbases aren't going to be that interested in watching the higher division. Right now Big 12 and ACC fans are watching plenty of Big 10 and SEC games, largely because those schools play in the same division and what happens in those leagues can affect your team. If that goes away, so do those viewers. And the same thing happens to a lesser extent with Big 10 and SEC fans watching Big 12 and ACC games. If all of a sudden those schools are all in different divisions where they aren't theoretically playing for the same thing, the amount of viewership outside of one's interest level will go down. Yeah, the Big 10 and SEC have enough fans to be economically viable without those other fans tuning in, but it's still going to mean lower ratings than what they have today, and there will be no tangible gain as it's not like the Big 10 and SEC are currently sharing anything financially with other leagues. They're getting a ratings boost from the rest of the country without having to give them really anything in return other than being involved.
On top of all that, these games are going to be on TV. Even a bottom tier ACC/Big 12 matchup is going to draw more eyeballs than any other programming those networks could put into those slots, and it's cheap to produce. Why would these networks intentionally devalue their own product to that level?