You may have misread the numbers slightly; population of ~117 million in 1980 and a population of ~ 125 million today.
And the chart in that article shows that they basically stopped having children in 1980 which would be consistent with a "running out of resources" hypothesis as a root cause. If they hit a wall in economic carrying capacity circa 1980 that chart would supporting evidence.
https://en.m.wikipedia.org/wiki/Aging_of_Japan