Just like NYC, the “big coastal cities” in CA are major points of entry for immigrants — that’s what makes them interesting and great. I don’t know if replacement via immigration is a new phenomenon, and there’s nothing much wrong with it.
As far as the rich in-migrating, this is a good thing, no? Builds tax base. And any concerns about taxes tend to be trumped by CA being an amazing and beautiful place to live once you have a certain amount of money.