Of course they could always dump the burden of the elderly by making death of elderly patriotic
That solves half the problem.
A steady population of 60M could be just fine with a 2.03 fertility rate. The Japanese could be happy being an irrelevant outpost in the suburbs of Beijing kowtowing to the Han Emperor.
A population of 60M with a 1.4 fertility rate on a downward spiral is still a march to Japan’s suicide. A better lifestyle on the way to suicide without having to pay all that elderly care for sure.