1) There is a big lag time for some crops to grow. Something like corn is no problem. But apples or anything else grown from a tree? Or any other crop that requires years of growing before it's productive? It will be many years until the California output could be matched.
2) Other places like the Midwest have potential water issues too. For example, look at the history of the Ogallala Aquifer. If a greater strain is put on the aquifer, it's possible they will also have the same water shortages that California has now.
The California water crisis is a national issue, but the rest of the nation would rather just laugh at us. The western states should be discussing serious proposals to build more aqueducts or pipelines to divert water from water-rich states to California. Maybe the Federal government could help with financing desalination plants. But this kind of cooperation isn't happening. I'm worried that what will happen is everyone will continue to just laugh at us until they go to the grocery store one day. Then they will care, but by then it will be much too late.