I live in Japan too. Sure there's very few trees in cities and more would help, but at the same time it's so hot that I doubt this would make cities significantly more livable in summer. We're not talking about some moderate summer heat like 33C low humidity. It's 36C high humidity in the afternoon and getting higher with climate change. In summer, the outside during daytime is for going from some place with AC to some other place with AC. Some people have to work for long time outside and they need something significantly stronger than a few trees to protect their lives.

With that being said, I agree some more trees would be nice.

Pus, they cut down large trees (or trim them to a stump) en masse... because of an invasive beetle or fungus? No. Because the leaves are annoying to clean up in the fall. Because the branches are "unsightly." Because it's make-work. Who knows? I see more and more shade-giving trees get stumpified every year.

And Japan's history of planting trees is... mediocre at best. They planted a lot, but pure (now not only useless {no lumber value} but harmful due to pollen fever) monoculture.