Correction: Yormark joined the Big 12 after the expansion of BYU, Cincinnati, Houston and UCF saved the league. He had no hand in keeping the conference together as that was set.
The only thing he did was reup their media rights without taking it to market, which did hurt the Pac-12 as they expected the Big 12 would play the market and up the potential revenue.
But yes, the Pac-12 killed the Pac-12. It was pure hubris from all those involved.