That logic doesn't work. 95% of new units going to existing residents doesn't mean that if I built 100 new units, I only get 5 new houses. Those 95% who move to new units leave their old local places open, which in turn go to god knows where.

There is a natural population growth in the bay area from people having and raising children here. Our housing growth rate is slower than the rate of children coming of age. The unoccupied units that current residents are moving into can theoretically be occupied by the children of the current residents as they come of age.