>Japan is vastly better situated than South Korea or China for what's about to happen
Depends how better situated is defined.
Advanced economies like JP, SKR or TW (and most of EU) with relatively maxed out human capital (~80% skilled workforce) are undergoing net demographic decline that will stagnate/contract their economies while accumulating increasingly onerous welfare commitments.
Versus PRC, whose only at ~25% skilled workforce, with lots of poor before rich aging cohorts that will be replaced by more productive newer generations, still has vast development + economic headroom even with less net population. Meanwhile poor before rich cohorts = expectation of welfare less demanding - many do not expect much in the first place. Hence most poor in PRC have high homeownership and household savings rate to weather the lack of anticipated state support (96% in poorer rural regions). They're vastly different scenarios to manage politically - average PRC retiree is going to have to suffice with much less vs retiree from an advanced economy, but that's expected from old before rich societies. Or just plain not rich societies, i.e. most societies throughout history.
But where PRC is vastly better situated, than just about anyone, including immigration heavy advanced economies, at any time in recorded human history, is PRC workforce trending from 25% skilled workforce to 70-80% over next decades - generations of high skill demographic dividend and high-value economic development is JUST starting in PRC. Quick napkin math, PRC will be recomposing the current 750M workforce with ~200M skilled to shrunken 700M workforce but with 450-550M skilled (+250-300M). For reference, current JP is like 55M skilled out of 70M workforce, demographic decline will lead to economic contraction. Also consider 2000 PRC GDP ~1T @ ~2M STEM grads, 2022 GDP ~18T @ ~17M STEM. Projected 2030 STEM @ ~50M, likely topout ~80-100M @ 2050. That's 4-5 extra current JPs worth of growth left, which will increase capacity to weather demographic decline from baseline, which might still be low relatively, but again, that's an entirely different demographic/political situation to engineer around.
The TLDR is many advanced economies are in net demographic decline with increasingly onerous welfare requirements, vs PRC has massive high skilled demographic dividend to buffer net demographic decline from old/poor cohorts on their way who also has savings to retire meagrely because they simply don't expect much from state in the first place. IMO person is better situated to retire in the former, but a governance better situated to manage in the latter.