CDs vs. NIMBY (betonit.ai)

But why does the high density housing have to be where the SFH are? Why does it need to share the parking, facilities, retail, roads, schools, etc. of something that was built and has been maintained and evolved for 50-100 years? Why not build the high density with it's own transit, roads, malls, parks, schools etc?

This is how it's done in the rest of the world, certainly Korea, Japan, and China. I see this in England and France as well. They develop a high density area with new facilities that can grow and evolve for how people want to live there, rather than mowing down houses (in communities of color) or plopping 5-7 story apartments without parking or parks right next to single story SFH, as developers like to do in the US.

I assume it's because it's more profitable for the developers. They don't have to build anything, but the part they can immediately sell, and they ride on the value of the neighborhood created by the people who already live there. I think some infill near commercial districts or malls also faces much less "NIMBY" criticism.

To give a more personal example: I like the location and view of where you live. I like the plants you water, and you keep the house looking clean. I want to just build another large room on top of your house, where me and a bunch of friends will live. We'll just share the stairs, hallways, and front door, and of course our share of the water and electric bill. We probably won't be too loud most nights, and usually when friends come over they won't use your bathroom or take anything from your rooms, or at least we'll ask them not to. Sorry about the construction noise while we do it, and just think you'll get to meet so many new people! Sorry that the part of your house you still own will be worth less than your neighbors when we're done, but that's the price you need to pay for living in a nice place and keeping it up.

The argument that SFH infrastructure repair is subsidized by new building is a much better one. Of course that was true of the tax increases and subsidy of the original town, when the SFH were built as well.

Can you show me an example where prices went down (in the long term)?

the problem is that all supply is not equal. in the classic NIMBY case, we’re talking about single family home owner in a neighborhood that has high housing demand, meaning it is largely built out, and they are opposing the introduction of multifamily housing to their neighborhood. in order for new multifamily units to come online, this means that over time existing SFH units are being converted/redeveloped into multifamily homes. this means that at the point where NIMBY pushback starts to matter, the number of SFH in a neighborhood has already largely peaked, and that the supply of that type of home will only ever go down and become more scarce (especially as a percentage of overall units). as more people move into a neighborhood in new multifamily buildings (and bring jobs, businesses, services, and general opportunity) the demand for that neighborhood goes up as well—demand is not fixed, and often goes up with supply, and depending on other macro factors can even outpace it if the agglomeration benefits are working properly. in this case, the NIMBY’s SFH value goes up, because the land value is much higher due to redevelopment potential, and also because that type of home is scarce and highly desirable to wealthy people who want proximity to opportunity in an urban place with the privacy and luxury of a detached SFH.

tl;dr: “while demand remains steady” is doing a lot of heavy lifting that doesn’t really make sense in a real dynamic urban system.