That's fair, but the stereotypical immigrant isn't near retirement. Positive migration tends to be related to youth, and it's hard to imagine that this will work for most.

Which, fair, it's their right to close the door fully, but lack of immigration and a greatly aged population sounds like a recipe for collapse.

The requirement is that your projected pension benefit, at your current income level, needs to match the average pension benefit for someone who worked for 30 years. And if you don't meet that requirement, whatever shortfall you have can be offset with an equivalent amount of savings.

It does not mean that you need to have 30 years of savings right now. Just that you are on track to get there by the time you retire.

There are a lot of people who won't qualify (and while it certainly sucks for people affected, this is by design) - but those who earn more than the Japanese average should generally qualify.