Japan has relatively low wages compared to Europe & the US, women work at a much lower rate, the birth rate is substantially lower even once you exclude immigration, gender roles are much stricter and opportunities for women outside of the house & child bearing are much more limited.
With all of your facts wrong, I'm fairly confident your conclusion is wrong also.
Even the worst countries in Europe have a higher rate than Japan, and most are substantially higher. Those numbers are exponentials, so a even a seemingly small difference is quite large.
The list I linked shows several places with rates worse than Japan, including Italy. Then there are places like Spain with rates that are about equal. Then there are countries that are barely better, like Greece.
I'm not sure where you're getting "substantially higher" from.
With all of your facts wrong, I'm fairly confident your conclusion is wrong also.