We wouldn't be "picking up every school we think the PAC is interested in". We'd be picking up the only attractive school still available to the PAC. A school that brings an excellent football program, fantastic facilities, one of the larger markets in the nation, and a presence in southern California. But most importantly, as Jon Wilner just told Chris, it ends the PAC and that will cause numerous PAC schools to jump to the Big 12.
Is it guaranteed to be a great move for the Big 12? No, but it's a calculated risk that I believe will pay off. The Big 12 can't afford to sit back and play cautious and still expect to survive and thrive.