If I had a duplex, I'd have a partner or friend registered as the occupant of one half and use both as short term rentals to subvert annual short term rental limits and the proposed vacancy tax. The vacancy rate probably doesn't tell the whole story.

I generally agree, SFs problem is 100% supply. But I think that ends up being caused by slow approval processes and highly restrictive tenants rights. Why would you ever subdivide your property in SF to add another dwelling unit? You can't give a tenant any amount of notice to end a tenancy. You can't ever merge those units back together. SFs property market is really crazy in this regard: older buildings with lots of units are often worth less than large single family houses. Properties with tenants sell for 25-50% less than vacant comparables.