I don't see where the problem is... The vast majority of countries, even many western countries only wish they had Japan's 'problems'.
Japan does a lot of things exceedingly well but demographics are one of those forces of history that are dammed near impossible to mitigate or circumvent without enormous cost, like trying to stop a train with your fist.
EDIT: Just to be clear, my point is about demographics, not immigration. We have yet to see if productivity advancements such as robotics can fill the void, but either way, Japan has a significant struggle ahead of itself.
Citation needed. Sure fertility is well correlated with educational attainment which is likely somewhat "heritable" by proxy (social class, intelligence, lack of disabilities) but that doesn't mean the heritable effect will be large enough in aggregate.
While Japanese economic progress postwar was astonishing, it was entirely premised on them having favored status in the region. Few people know, for example, that the US forced Korea to purchase massive amounts of goods from Japan to rebuild its economy, essentially reinstating their former colonial relationship and stoking tensions that culminated in the Korean War.
Japan is a cautionary take because it was entirely too dependent on using its favored status in American geopolitics to throw its weight around Asia-Pacific, as opposed to building meaningful, trade and immigration agreements with other countries in the regions. Now that US dominance is in retreat and the Chinese ascendant, they’re in a poor overall economic position to counter them.
Certainly the geo-strategic position was favorable, but I think this is quite the exaggeration.