What did the old do when they couldn't hunt and gather anymore ? If they didn't die, the young took care of them.

People who are old stop contributing to society but they don't stop taking. Someone has to make up for that, doesn't matter what system.

> People who are old stop contributing to society

You've never used or heard of grandparents being used to provide childcare?

Anyway, providing we can improve productivity sufficiently, then along with various measures that will enable the elderly to largely take care of themselves (medical advancements etc.) I don't think we're going to reach a situation where we simply have no capacity to keep the oldest generation alive and well. I can also see laws and attitudes towards euthanasia changing such that those with the highest care needs will no longer expect to be kept alive regardless of quality of life.

Grandparents too old to work are also too old to provide actual childcare.

There will always be some percentage of the population that are essentially incapable of contributing further to the economy due to advanced age, and yes, it will grow, but I suspect you underestimate the vitality of many older Japanese.

It's simple physics