It isn't all bad. The aging population will have to be supported by fewer people, which is obviously a problem, and one that can really only be solved by asking the elderly to work to a greater age.

On the flip side, Japan has incredibly dense cities with tiny houses. As the population drops to half its current level there will also be more space for living decently, with bigger houses, more green spaces, etc. It's not at all impossible that in such a situation, women may choose to have more children again.

At least there will be fewer people in Japan suffering from the climate changes over the next century.