and the way to do that is to up-zone

Up-zoning doesn't guarantee you cheap residential. A lot of highly dense cities are still very expensive (NY, Paris, London, etc.) and will continue to be as long as buying and holding the property is one of the best long term investments one could make.

These places would be even more expensive if they had not up zoned.

All else being equal: More supply -> Lower price.

Japan only really started doing that as their population began to fall. It's a situation lots of Central Europe is starting to experience too. Before then, the land the Tokyo Imperial Palace occupised was worth more than (theoretically, at prevailing market rates) than all of California.

I'm not sure I buy that simplistic argument. As a counter-example, residential property in most exUSSR experienced significant price increase during previous 30+ years while populations of those countries mostly decreased during that time (and often much more significantly than in Japan). Why? Because people were (and often still are) convinced that it is best investment for them long-term.