> But for a property owning corporation it is obviously a poor idea to invest in new apartments because that would reduce the value of the ones they already got.

That doesn't seem sensible to me. One additional unit of housing will reduce prices by some very small amount, but rake in quite a bit of cash for the company owning it, in the current housing climate in SF. Closer to some kind of sane equilibrium, those numbers might start to balance out, but right now they're pretty far apart.