Didn't say it was perfectly fungible, just fungible enough that a significant enough increase in other types of supply might bring down SFHs. Either that or the true value of a SFH in a no longer housing constrained San Francisco really is $2.5 million.

If you believe that SFH buyers do not insist on SFH neighborhoods (whole cities are not neighborhoods), and should welcome denser zoning because it makes their lot more attractive for redevelopment, go circulate a petition for this among SFH owners and see how far it gets.

You're thinking far too narrowly on SF. In SF, there are people sharing a rented house and treating it as de facto apartments, so my argument still holds. Outside of college towns, that is normally a rare thing, so it isn't applicable for most metro areas that haven't been frozen in time for decades. The Bay Area is a terrible example because they've banned ALL types of housing, not just apartments. They have an urban growth boundary and haven't upgraded their infrastructure to support their growing population, which means that for a given commute time, people must live closer to work than they would with better infrastructure, which increases the number of bidders for each property near offices. I suspect though that even if enough apartments are built in SF so that single-family homes become single-family again, the price of a house there will still be millions. For example, look at the Upper East and West Sides of New York, where there are townhouses worth millions to tens of millions, just like there are in SF, but they are surrounded by massive apartment buildings. Those townhouses are also protected by zoning (In the local government's words: "R8B contextual districts are designed to preserve the character and scale of taller rowhouse neighborhoods."), and they'd be worth even more if a skyscraper could be built there. You could argue NYC still needs more apartments there, but the value of that townhouse's land would still be high, as the developer who wants to build another skyscraper with hundreds of units can afford to pay far more than all but the very richest potential homeowners. A regular New Yorker who wants a house, while they may be priced out of Manhattan, still has the option of moving to New Jersey or somewhere on Long Island and commuting in, as NYC sprawled in addition to building up. If they want to stay a homeowner in Manhattan (or a San Francisco) then they have to outbid the developers.

To your other point about insisting on SFH-only neighborhoods, in large parts of the country, there are apartment buildings and large commercial areas spread between and in single family neighborhoods, and that hasn't stopped people from buying houses there. I wasn't arguing that they all should support higher density zoning, just that it is in their financial best interest to do so. Another major problem is that because the Bay Area has refused to build anything for decades, it has decades of unmet (or to use urbanist language "induced") demand for houses, apartments, roads, transit, etc. that has to be met before prices and congestion will start to go down. To maximize housing affordability you need a mix of sprawl and density with appropriate infrastructure for the type housing built, and if you only do one type of growth you will have many people who are unhappy, which is why in another comment in this thread I accused the YIMBY urbanist of being the same as a NIMBY SFH owner, just with a different preferred housing type. The Gallup survey I linked earlier shows that there are more people currently living in cities (presumably in apartments) who wish to live in suburbs or rural areas (presumably in houses) than the reverse, so there is an unmet demand for "sprawl" and options like remote work.

My major point if a SFH owner in SF was purely motivated by money, they would welcome development on their land and wish to limit it on others'. Many of them aren't though, and they aren't lying or using euphemisms when they say they want the character of their neighborhood preserved. I hear tons of arguments that they are opposing multi-family housing because it would lower their property values, and that just doesn't make sense. A San Francisco with an apartment built for everyone there who wants one and no other changes would most likely still have million dollar houses, though the rent of the apartments would be less.