5-ND stays independent because NBC is going to give them Big Ten money and let them remain independent. There's a reason they hired NBC Sports' Chief Executive to replace Swarbrick.
If I were the Big 12, I make a big swing at Washington/Oregon. Even if you know their end goal is the Big Ten, you might as well be the ones to make money off of them while they're in limbo. Add Washington, Oregon, Colorado, Arizona, Arizona State and UConn and be done with it. If/when Washington and Oregon leave you still have a very solid 16 team league to fall back on.