Yes, you're right. And that's one of the most ironic parts of the SF development debate: the folks who complain about "NIMBY opposition" the loudest are (usually) talking about already dense neighborhoods, like the Mission, when they should be thinking about the Outer Sunset or Laguna Honda.

That said, it doesn't change my point: to change the density of these places, you have to buy out a bunch of people who know exactly what their land is worth if it's zoned for higher density. Many of them are actually betting on that windfall. There is no equivalent in SF to the vast tracts of undeveloped land in, say, Texas.

That is an excellent point, however, part of the reason those areas are not as densely developed is that the transportation infrastructure sucks.

And how to fix it? It takes a long time and a lot of investment to improve public transport, but it's challenging when the need for it is driven by short-term boom-bust tech "bubble" cycles. Makes it harder for the city to invest when demand might go poof in a year.