So over 20 years, your property taxes would increase by ~49%. The median sale price in 1996 for a home in SF was $275/sq. ft, today it's more like $1,250/sq ft. 450% appreciation in 20 years, or something like 7x faster than property tax increases.

And why exactly would that help housing availability and costs for people living in SF?

Supply side market pressure. More housing, lots, and lost, and lots of housing. Also, corresponding infrastructure (including civic utilities like parks) to accommodate the additional residents.

Which is insignificant compared to yearly property appreciation in the same area.

Let's please test your intellectual honesty, shall we ?

You are indeed correct that in our very modern era, the 2% ratchet under prop13 is less than what we see California real estate appreciating at.

So would you indeed support rebates and/or clawbacks and/or negative property tax increases if California was to ever see sustained drops in valuation ?

I own property in California as well as in other states and I can tell you that the property taxes sure go upwards very easily ... not so much the other way around ...

I don't think the people railing against prop 13 have ever lived in states where property tax rates are out of control to the point the people lose their homes... it's not pretty.

Do they lose their house in the sense of selling it at a massive profit? Or are the taxes rising irrespective of property prices?

I'm not sure what it used to look like in CA pre prop 13, but I get the sense that many people lose homes due to even fairly short periods of cashflow problems, leading to tax leins leading to forclosure (and possible loss of much more equity):

http://money.cnn.com/2012/07/10/real_estate/tax-liens/

Seniors in those kind of conditions, would be unlikely do better selling off their homes and subject themselves to a more volatile rent for their homes.. which would likely lead to even higher societal financial supports needed so seniors don't end up on the street.

I see sources backing up percentage increases that are much larger than 2%

http://www.zillow.com/san-francisco-ca/home-values/

https://www.boe.ca.gov/ma/news/AV_Bay-Area_PC.htm

Sales reset the property taxes on properties. It was one of a few reasons I didn't buy a home in the Bay Area a few years back. Probably should have, my buddy that bought his house for 500k in the city is sitting on probably 1.2-1.4m now. Not bad for 3-4 years appreciation.