This article argues that Japan’s postwar housing shortage and today’s vacant-house (akiya) crisis form a continuous policy story rather than two unrelated problems. In the immediate post–World War II period, war damage, evacuation-related demolition, and repatriation generated an estimated net shortage of roughly 4.2 million dwellings nationwide, creating a political imperative to maximize supply as quickly as possible. Government responded by institutionalizing a “build-more” paradigm through direct provision and finance—public housing, public corporation development, and state-backed mortgage lending—supported by national long-term construction planning. These instruments were highly effective under conditions of rapid urbanization, population growth, and household formation. However, the same system produced strong path dependence: it normalized new-build solutions, reinforced owner-occupied asset logics, and locked housing supply to infrastructure footprints and land-use patterns that are slow to adjust. As demographic and economic conditions reversed, surplus emerged. By 2023, Japan recorded 9.0 million vacant units and a vacancy rate of 13.8%, with a substantial share categorized as non-market “other” vacancies that are difficult to circulate or repurpose. The article proposes a future-oriented shift from “quantity of new supply” to “operations of existing stock,” emphasizing low-friction mobility (downsizing and near-distance moves), design-for-reconfiguration (flexible plans and adaptive reuse), and area-based “reconnection” that bundles vacant homes into walkable service ecosystems aligned with compact-city strategies. Reframing housing as an editable, interoperable system can turn the legacy of postwar expansion into a platform for resilient living in an era of population decline. This perspective links reconstruction history to smart-city governance and community-scale management models today.