We are witnessing a period of dramatic global change – from geopolitical conflicts and rising defence expenditures to climate change, biodiversity loss, and the structural transformations of the knowledge economy – that places mounting pressure on ageing welfare states. In this concluding chapter, we situate these challenges in historical and comparative perspectives while critically reflecting on the results presented throughout the volume. The chapter evaluates the strengths and limitations of the social investment-intervention (SI+I) framework, asking whether the integration of investments and interventions can provide the flexibility, effectiveness, and legitimacy required to sustain welfare states in the twenty-first century.