In some ways, I can see what you are saying.
Japan is in a very different position than Western countries such as the US or Canada.
They have a very small % of non mountaneoues land suitable for city building. Add to that their inability to build tall buildings and the huge population, and it is quickly apparent that Japan could do with fewer people in the country.
Now the obvious problem of having an aging population is collapse of any system that depends on the young to fund an aging generation that is now a net burden for the government.
Option 1 :
When immigration is not an option to artificially ramp up net contributors, the population will have to come to terms with an uncomfortable reality. That, the aging population can't avail the benefits they were promised when they worked their asses off, because the smaller young population can't bear their burden.
This ofc, is extremely difficult to implement in practice, because older people are big on voting and will destroy any politician who even suggests this. But, the other alternative could lead to an even worse outcome than the mass migration idea.
Option 2 :
The aging population keeps drawing from the system at the cost of an increasingly burdened working population and the debt keeps rising. Eventually, debt will be too large and the one generation capable of putting a dent in it will be far too crippled by economic burden of now dying populace.
Your comment seems to be an eventual outcome of option one. Where government support for systems for the retired vanish, and again people start liquidating assets to make up for vanishing welfare. As a result, housing may become affordable and the younger generation could gain some much needed stability and options.
I am not an economist and the economic implications of any of these events go over my head. It will however be interesting too see an experiment where a country tries to maintain a stable economy and QOL for their population for a country with much older average citizen, than he norm.