The era of human-operated software is reaching its limits. As organizations struggle with SaaS sprawl and rising integration costs, enterprise architecture is undergoing a fundamental shift from renting tools to procuring automated outcomes. This paper introduces "Service-as-a-Software" (SaaSw)—a new paradigm where the user interface becomes a strict governance plane, and autonomous, multi-agent systems take over the execution plane. By introducing a formal Event Algebra, calculating ROI through Expected Execution Yield, and solving the AI liability vacuum with a Verifiable Execution Layer (VEL), this research provides a rigorous, contract-verifiable blueprint for the future of enterprise automation.