While Oregon and Washington would be the best available, it certainly doesn't calm things down. They'd be constantly discussed about joining another conference. The stability of the other 4 is better in the long run.
I mean, that's why you take all 6 if available, and if those top 2 leave down the road you're no worse off than if you'd just taken the more stable 4 to begin with. In the meantime you cash the checks from having those bigger brands in your media rights deal