Software assurance is traditionally treated as a collection of supporting activities—testing, audits, compliance reviews, and certification—performed around software artifacts and release milestones. While these activities remain necessary, they are increasingly insufficient for explaining or managing risk in modern software systems that operate at scale, evolve continuously, and integrate across organizational and technological boundaries. Many enterprise systems exhibit acceptable defect metrics and regulatory compliance, yet still experience sudden operational failures, trust erosion, or systemic breakdowns. This paper argues that such failures cannot be fully explained through defect-centric or activity-centric assurance models alone. Instead, they reveal a deeper limitation in how assurance itself is conceptualized within software engineering. Treating assurance as an episodic activity obscures whether confidence in system behavior remains justified as systems evolve, architectures change, and operational contexts shift. To address this limitation, the paper proposes reframing assurance as a first-class system property, comparable in importance to reliability, security, and performance. Under this perspective, assurance is not an external overlay applied to software, but an intrinsic property that emerges from the alignment between actual system behavior, validation coverage, governance mechanisms, and the temporal relevance of assurance artifacts. The paper introduces a conceptual framework that explains how assurance properties develop, degrade, and diverge over time, distinguishing between assurance-aware systems that actively maintain justified confidence and assurance-fragile systems that rely on historical validation and inferred trust. By elevating assurance to a system-level concern, this work provides a foundational lens for reasoning about latent risk accumulation, trust erosion, and resilience in complex, continuously evolving software systems. It also lays the groundwork for future research and governance practices that treat assurance not as a procedural obligation, but as a sustained and evolving property of software systems.