https://www.imf.org/en/Publications/fandd/issues/2020/03/shr...
It's not a unique Japan problem tbf. The young people are getting shafted all over the world.
If the issue is wealth disparity, then why only target the elderly not the young people who are wealthy as well?
And the elderly largely don't work, because they can't anymore.
Shifting services means the elderly will get poorer. Which given the high wealth level they have in developed countries relative to the young is not a catastrophe. However it would be a huge adjustment to expectations downward, since the elderly would have the rug pulled out from under them. Whereas the young haven't yet had a chance to adjust their expectations up since they haven't had money yet. Hence the political settlement.
For a prosaic example, in many US states funding for public universities has been cut to pay for public sector pensions in the past thirty years. This has shifted educational costs to the young in exchange for maintaining the benefits of retirees. This is an example of shifting services from young to old.
The actual amount of goods and services produced will be lesser and/or be stretched thinner.
The only real way to combat it is to import young people from places with a mostly young population and getting them productive to make up for the productivity shortfall.