The problem is that this is very uncharted territory - socially, economically etc. Japan is not a small country - it still has 125 million people and it is fairly wealthy by global standards.

A "home" cannot be thought of simply as walls, rooms and a roof. Think about all the other things that come with it - roads, electricity, water, sewage etc. You'll need shops, hospitals, police, government etc to support a community. So you cannot take a simplistic view that all of this "comes for free" or "will always exist".

Infrastructure and social services need a workforce. Power stations grow old and need maintenance and replacement. Roads don't repair themselves. People need to eat. People get sick and get into accidents.

How does a large country provide for this if 40% of their citizens are retired and need additional care? Can the working population grow enough food, make enough product, etc etc if this happens?