Artificial Intelligence is often presented as a shortcut to organizational sustainability, yet many AI initiatives underperform because firms attempt to automate before they have structured, governed, and shared their knowledge assets. This paper addresses that gap by asking how organizations can move from fragmented, tool-centred AI adoption to sustainable knowledge ecosystems. We build a design-oriented conceptual framework through analytical generalization from four design artifacts previously developed in the legal-AI domain: the knowledge commoditization paradox (Allega, Puzio and Rizzo, 2026b), the MERL-T multi-expert architecture (Allega, Puzio and Rizzo, 2026c), the RLCF governance model (Allega, Puzio and Rizzo, 2026d), and the ALIS platform framework (Allega, Puzio and Rizzo, 2026a). The legal domain is treated as a revelatory setting because it concentrates stringent requirements for traceability, pluralism, accountability, and knowledge integrity that are increasingly relevant across knowledge-intensive organizations. The paper makes two main contributions. First, it extends the earlier notion of semantic entropy by identifying three entropic pressures that undermine the Knowledge Journey: coordination entropy, generated by fragmented repositories and inconsistent taxonomies; semantic entropy, generated by the decontextualization of knowledge during computational representation; and governance entropy, generated by opaque AI-mediated workflows that dilute authority and responsibility. Second, it translates these pressures into five design principles for sustainable knowledge ecosystems: commons-first knowledge architecture, provenance-preserving services, epistemically differentiated human-AI collaboration, community-anchored governance, and sustainability-by-measurement. These principles are then mapped onto the three phases of Planning, Acquisition, and Impact to show how organizations can reduce hidden costs such as search time, duplication of effort, hallucinated outputs, accountability gaps, and knowledge lock-in. Rather than treating AI as a stand-alone capability, the paper argues that sustainable value emerges when architectural choices and governance mechanisms are designed together. The result is a practical and theoretically grounded model for organizations seeking to transform knowledge management from a hidden cost centre into a resilient, auditable, and sustainability-oriented ecosystem.