It doesn't have to. Housing can be affordable while going up in nominal terms.
Neither do I think most San Francisco homeowners think about housing supply strategically. They're pissed off about a specific view being blocked. Or construction noise. Or their area's average rent going up. (?!?) Alternatively, they're comfortable and can't imagine someone might be better off housed in worse conditions, cheaply, than struggling to make rent in a decked-out flat.