> But why does the high density housing have to be where the SFH are?

It depends on where said SFHs are. Cities need to eventually grow upwards! If we never allowed land-reuse Manhattan would still be 2 stories on the whole borough.

I live in Toronto and the harshest NIMBYs are adjacent to downtown and/or subway stops. They fight tooth and nail against even 6 story condominiums on the major high streets (Margret Atwood, who lives in a large SFH close to downtown was among them recently-ish). The ultimate result is that we have a handful of extremely dense clusters of condominiums centred around a few subway stops that then super-concentrate people and cause extreme crowding on transit, amenities, etc. It would be better if you had mid-rise multi-unit dwellings more spread out.

It's actually become an acute problem in so many ways. The people who own the SFHs tend be over 50 and childless. The result is that many schools in these neighbourhoods are at barely 20% capacity, while the condominiums (where most younger families can afford to live) are causing the schools near them to be overflowing with kids.

> 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.

The countries broadly speaking default to higher density than North America for historical and economic reasons. They have their own issues, but the majority of Koreans live in high rises and Japan rebuilds their buildings constantly: https://www.theguardian.com/cities/2017/nov/16/japan-reusabl...

All of Tokyo was basically rebuilt after the war, because it was bombed. Much of it was not originally built to earthquake standards. Rebuilding high density in bombed US SFH neighborhoods, would certainly make sense.

I completely disagree with broader point of "cultural reasons" why Asia built high density, because it is all so recent. Mostly in the 1970s-2010s. They built entirely NEW infrastructure, with transit (e.g. subway, highspeed rail), malls, streets, commercial and residential buildings. Look anywhere in Korea, look at the rise of Shin-Yokohama (outside Tokyo), look at the highrise Urbs outside of Paris (out all the way to Rennes).

Why is the center of Paris still only few stories high after the Hausmann era when it was laregely built over 150 years ago. Why don't they put 50 story skyscrapers in Paris... because they don't want to, and they push the "density" to the edges where new facilities can support them. But the housing prices in Paris are astronomical (just as they are in London).

If you want to talk about closing schools, because there aren't children for them... then great! Build new high density where the school used to be. Build a school into the building, put a subway under the school with a connecting mall. Then at least the surrounding neighborhood that has been paying 70 years of taxes doesn't have to deal with the extra traffic/parking of the new construction. Maybe they'll even enjoy the new mall, and foot traffic will help support existing commercial infrastructure.