I can't speak for everyone, and this is just my personal experience. I grew up in SF, and I haven't seen this sinister quality in people who oppose density. I disagree with them, I'd like to see SF add more density, but my impression is that most of the old timers in SF oppose new development because they are resistant to change and highly suspicious of developers, not because they want to create shortages and spike the value of their property.
In short, at the extreme, the school teacher or SF state professor who was able to support a family and buy a 4 bedroom house in the sunset in 1975 wants the new professor in 2016 to be able to buy a a 4 bedroom house in SF and support a family, and is unhappy that this is no longer possible.
They also tend to believe, rightly or wrongly, that "density" would just mean lots of high end expensive apartments that nobody in the "middle class" can afford. In short (and again, I'm guessing), they are very cynical, perhaps irrationally so, about what development will do.
There are, of course, some pretty horrendous examples of redevelopment projects and so forth in SF. I know, you're already reaching for the keyboard to explain the difference, but you'd be preaching to the choir here. I'm just trying to explain why it's not really a desire to maximize the value of a real estate asset that drives so much of opposition to new housing and development. The whole thing has a very bad reputation in SF.
There is such a thing as YIMBY in San Francisco, often people who have seen their friends and children priced out.