The median price of homes for sale today in California is $500k. So even if you put 0 down (average is more than 10% down of those with mortgages, and ~30% are paid full in cash), less than half of NEW mortgages > 600k. So I'd guess less than 10% of those in California have a mortgage over $600k counting mortgages before this year. In LA and SF, those #s are higher, obviously.
Again, why should we be encouraging people to take on debt (mortgage) or encouraging high taxes at the local level?