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.
You don’t think the population would turn around if they young ones wouldn’t have the elderly to support?