While it might be true that everyone has a price, if we plucked someone off the street and they told us they never wanted children (at all, or more than they have now), how much money do you think it would take to persuade them to have a kid?
Do you think it would be $1000? How about $4500? Maybe it costs a whole $12,000 right? These are the sorts of incentives that are offered in Europe, in South Korea, etc. They don't seem to influence much extra in the way of births. And it's not difficult to see why... those people are told (whether true or not) that children are far more costly than those sums. So we're still talking about it being net negative.
In some publications, people in the western world are told that it's some large fraction of a million dollars to raise a child to adulthood. How many babies could Japan afford, if it had to pay parents $500k for each?
It's even worse than that though. Many Japanese women of child-bearing age aren't even in circumstances where it is plausible for them to consider having a child. No husband, or a husband whose career doesn't make being the sole provider possible. Little chance of those circumstances changing before motherhood is out of the question. Etc.
What the Nordics in general do well, and Germany does ok, is preserve the ability of mothers to have careers, thus making motherhood a bit less of a drastic decision (it’s still a drastic decision. You’re committing to unconditionally love someone who will, say, bite you hard on the shoulder because you had the temerity to suggest it was time to go potty instead of play with trains, and to take the physical damage of pregnancy that has so far led to my first broken bone and back pain that never quite goes away)
But I’m still working in my pre-maternity department, in a scaled-back version of my old job, and more critically, keeping up with our industry about as well as my not-mother colleagues, so once the little nipper can escort himself home from school, I can more easily go back to full time.
And in case my husband loses his job that currently is our main source of income, we have the backstop of my job and the potential to go back to my full time IT income.
This would be far more difficult in a country where my large employer was not obliged to let me work part time for several years, and Elterngeld didn’t make taking a year off after the birth of a child fully expected and planned for by employers. My husband was also able (and expected) to take a month off after the birth, and then another when I went back to work. A lot of men in relatively conservative Bavaria were initially hesitant to take those two months, but it’s now normal. We’re not at the point that it’s normal for most fathers to also exercise their right to switching to a part time schedule for their children’s first few years - no idea if this happens more often in Berlin or Hamburg.
Germany has its own demographic problems, but not as severe as Japan’s.
But, even if that could help, the cultural changes Japan requires to make that possible, just aren't feasible in fewer than half a dozen generations. Which is sort of what they're running out of anyway.
Germany will get to where Japan is, and it will be within our own lifetimes.