California kids have been leaving the state in droves to play elsewhere. The best ones go to the SEC, B10, Oregon, other P12 states,etc.
USC/UCLA in the BIG means a lot of kids who might have left the state for greener pastures will find them in a B10 that includes CA schools. So that will further hurt the remaining PAC schools in recruiting. All the CA kids who want to stay in the west will look to LA first.
The PAC may take their time and stay together. But that conference will become a shell of its heretofore mediocre self.