Beyond High Recycling Rates: Aging, Legacy Material–Stock Release, and the Quality–Capacity Frontier—A Normalized Scenario Analysis for Japan

Japan’s shrinking population lowers current material demand but may accelerate retirement of inherited buildings and infrastructure. We develop a normalized two-material vintage stock–flow model that separates recovery, displacement quality, domestic retention, processing capacity, and downstream outlets. Under a stylized 95% mineral recovery target, the reference quality threshold is 0.374. Along the 2025–2070 demographic path, cumulative material-cycle emissions are 16.9% below a frozen-demography counterfactual; additional legacy release offsets 4.0% of gross scale savings. Advancing both quality and capacity lowers emissions by 6.55% relative to both-delayed preparation; a strict inflow-linked outlet reduces this difference to 2.97%. An illustrative concrete–asphalt–metal sensitivity changes the sign of the release offset in some designs. A restricted official output-composition weight combined with explicit displacement-value assumptions places constructed quality on either side of the reference threshold. Conditional cost-ratio comparisons and 0%, 1%, and 3% time weighting connect physical constraints to economic evaluation without estimating Japanese costs. We distinguish inventory emissions, cost-effectiveness conditional on package costs, and social welfare. These normalized diagnostics inform sustainable resource management. They are not national forecasts, facility validation, or investment recommendations; high recovery alone cannot ensure effective circularity.