No way that FOX will be content with just one big college football game, and probably not CBS either. As much as they shelled out for the B1G, it's still only one time slot on each network. Having the Big 12 serve as the second part of a double header on these networks should be compelling enough to keep folks from switching the channel.
I just think with the level of investment these networks are putting in to attract college football fans, it makes sense to have other games to keep the eyeballs on your network.
That could just mean BIG expansion, although if you're a FOX conference, it is nearly one and the same to the networks. More scheduling constraints, but also more efficient cost structure.
Imo, if you're trying to have more inventory and make the other slots as valuable as possible, you'd want to take the best of the rest and put the BIG (or SEC brand when talking ACC), have those games matter more to the biggest collective of fans. UW/Oregon/Stanford etc likely help you pull more fans as matchups if as a BIG matchup.
Whatever they do to fill, it has potential to cannibalize some the T1 spots they sold, which is why there were rumors FOX had little interest in PAC or Big 12. If you're going to cannibalize a BIG game, a conference that FOX basically has equity in due to BTN ownership stake, doing it with other BIG games makes some sense imo