It's hard to say how much of the Cartels' money comes from MJ vs other drugs, but California's proposal certainly won't help them.
Ultimately, federal enforcement will fail w/o the assistance of state and local authorities. They simply don't have the manpower or the mandate to make it work.
The notion that California growers won't expand their operations in the event of legalization there is a non starter. Obviously, they will, and the risks associated with transporting it to other states are much lower than getting it across the border, so the domestic market will obviously see more California MJ. State and local authorities simply don't have the same search and seizure prerogatives as Customs.
MJ prohibition has been and always will be a ridiculous waste of resources that would be better used elsewhere (like investigating financial crimes) so I applaud the efforts in California, and hope that their initiative passes.
BTW, I haven't had a toke since 1988, and don't plan on starting up, so I have no personal interest here...