I am currently living rent-free in Japan because there’s so many empty houses that just need looking after.
Everywhere in Japan has a declining population except Tokyo and Osaka, and both cities are well designed and don’t feel crowded. I think Japan is around the 50th most densely populated country, but the very well designed public transport systems and well planned (and mostly hidden!) highways make it so much more open and walk-able than many less densely populated places.
The only place that feels crowded is Kyoto and that’s because it was never built to sustain the level of tourism it attracts.
Japan has a limitless water supply. They don’t even charge for the stuff in many places (like here). The agriculture industry is strong and supplies most of the food.
The country is still like 70%+ untouched forest.
Japanese policy and mainstream culture is xenophobic and racist, mostly towards Asians (and indigenous people, and lower caste people), but starting to improve based on sheer desperation because of the declining population and economy. Unfortunately, few have the language competency required to work here, and basically no one here can speak English, which makes it really hard to attract people, as the government has not put in the support measures like Korea (a more densely populated place with an even lower birthrate!). It’s not uncommon to see Desi, Vietnamese and Thai workers in convenience stores in Tokyo now though.