But if you have denser cities, that leaves more room for rural areas. I don’t think anyone is arguing for less rural land, but as it stands, it is legally impossible to build another city like San Francisco or Manhattan in the US, and that’s a problem for sustainability. People want to live in those places, and not just because they are old.

The problem is not that you can't legally build a city like SF in the middle of nowhere. There are hundreds of smaller cities that could grow into bigger ones, but their population growth just isn't there. People wouldn't come to your "New SF" either.

The real problem is:

- Many people choose to live in big hubs like SF or Manhattan because that's where the high paying jobs are

- You can't legally turn SF into Manhattan (i.e. making it denser)

It's legally impossible to even build San Francisco into a place like Manhattan. Most the city is only zoned for two stories.

LA is zoned for a lower population today than it was half a century ago, and in that time the population has increased by 50%. Enter housing crisis.