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?

The issue is that keeping the elderly alive will mean devoting all energy and money to keeping them alive, causing stagnation. It's worth being concerned about.

People retire and lean on government services for healthcare and supplemental income - money that has to come from somewhere.

These systems rely on there being more workers paying taxes than retirees, and can’t survive when there are more old people than young people.

There are also broader geopolitical concerns - working age people form the core of your nations economic and military might. As that core shrinks, your nation becomes weaker in both a literal and economic sense.

For an island nation with a literally and economically hostile authoritarian power right across the pond, this is a grave concern.

The problem is already so bad that elderly people have been found dead in their homes, and rotting there for weeks, even months. They have a word for it, kodokushi.

The issue Japan has is similar to other countries in that they’re not replacing current people with enough people to keep the country operational. On top of this, and in no way am I saying this as to imply racism on my part (I used to live in Japan and love their country), but their immigration laws are extremely strict with a layer of xenophobia. The #1 way countries have alleviated demographic challenges in the past are through immigration. The US has done this and still does. For example, we import many farm workers, low-wage industrial workers, and scientists, too. When a country refuses to accept outside help, problems arise.

Currently there are over 100 million forcibly displaced people on the planet, 89.3 displaced globally and 53.2 displaced internally within their home country. That’s 1% of the population, or 1 in 78 people, roughly.

What a deficit of working-age people will do to a country we can already see some of the effects of now. Labor shortages, sometimes severe, which leads to increasingly higher demands put into current workers. Services and goods are less available because production decreases. The quality of things decrease, like maintenance getting done. It takes longer to get things. Some businesses shut down because it becomes impractical to keep enterprises going. That’s a brief explanation what demographic imbalance looks like.

An example of a country that has perfect demographics is actually Mexico. They have the right number of old, working age, and young people.

Another thing to consider is that there will be a higher ratio of retirees collecting pensions vs. workers paying into the retirement system, which would mean higher taxes on workers to pay for those retirees' benefits.

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