The population of Japan is projected to be 64 million in 2100. In 1930, the population of Japan was 64 million. Did it not exist back then?
To be fair you'd need to consider how the percentage of young people looked in 1930 for that to be a meaningful comparison to today
You should only be comparing the working age population or maybe even the ratio between that and retirees since absolute population is a deceitful metric.
Japan can easily operate with 60-80 million people in a country slightly smaller than California. And expand again in the future why not?