This article develops and tests a long-run growth proposition derived from the Economics of Belonging. Its central thesis is that domestic saving determines the scale of accumulation, but sustained access to the manufactured consumption of the global middle class determines whether that accumulation remains technologically valid. Such access can arise through a broad domestic middle class or through manufacturing value added ultimately absorbed by final demand in the United States and developed Western Europe. The theory therefore distinguishes frontier-validated growth from artificial growth based on protected demand, obsolete capital, natural resources, or post-collapse rebound. The empirical analysis proceeds in two stages. The first uses four initial cohorts from 1995 to 2010, OECD Trade in Value Added data, World Bank distribution and macroeconomic data, and Penn World Table productivity measures. Baseline regressions confirm that domestic saving predicts ten- and fifteen-year GDP-per-capita growth, while global-middle-class access predicts subsequent manufacturing growth. The stricter test excludes economies with insufficient access and relates initial conditions to twenty-year outcomes. Among developing industrial economies with an access measure of at least 15 percent of manufacturing value added, the standardized access-saving interaction is positive for manufacturing value added per capita, total factor productivity, and capital productivity, but not for aggregate GDP per capita. The second stage examines historical cases from 1960 onward. Soviet growth slowed sharply before political collapse despite high accumulation; independent evidence documents declining capital efficiency, a widening productivity gap, and weak world-market validation of manufactured exports. East German manufacturing productivity was far below West German levels before reunification and industrial production collapsed when protected demand disappeared. A separate low-access, low-saving cluster—Madagascar, Niger, and Burundi—records virtually no long-run growth per resident. China before and after opening, Korea, Mexico, and post-Soviet Russia provide additional contrasts. The evidence supports a qualified conclusion: access and saving behave multiplicatively for structural and technological outcomes within developing economies, whereas GDP growth alone cannot distinguish frontier-compatible development from artificial accumulation.