Access to affordable finance remains an important constraint on the development and sustainability of small and medium enterprises in Nigeria. However, the existing literature frequently treats credit access as a binary condition and gives insufficient attention to the structure of the loan received. This article presents an integrative review of the influence of structured soft loans on SME development and sustainability in Nigeria. Structured soft loans are conceptualised as concessionary credit facilities whose interest rate, loan size, repayment period, moratorium, repayment frequency, collateral requirements, disbursement timing and business-support components are deliberately aligned with the operational and cash-flow characteristics of the beneficiary enterprise. Drawing on recent literature on SME finance, financial inclusion, credit constraints, enterprise capability and development finance, the review argues that affordable interest rates alone do not guarantee productive outcomes. The evidence indicates that well-structured credit can improve business formation, working-capital stability, productive investment, employment, innovation, market expansion and resilience. These outcomes are nevertheless conditioned by financial-management capability, loan adequacy, timely disbursement, infrastructure, inflation, market demand and the quality of post-disbursement support. The article develops the Structured Soft Loan Sustainability Framework, which explains SME outcomes through a sequence involving financial access, loan–enterprise fit, productive deployment, enterprise capability, cash-flow generation, repayment and reinvestment. The framework introduces the concepts of loan-structure–enterprise fit, affordability illusion, repayment-cycle mismatch, support complementarity and finance graduation. The review concludes that public lending programmes should be evaluated not only by the volume of funds disbursed or the number of beneficiaries but also by enterprise survival, productivity, employment, repayment quality and the capacity of firms to graduate to sustainable commercial finance.