Mainframe
Computing was centralized, expensive, and controlled by a small number of vendors. IBM dominated. Organizations built deep expertise in proprietary platforms — JCL, COBOL, DB2, batch processing. The mainframe was not just a technology; it was an operating model. Reliability and throughput were the defining virtues.
Legacy batch processing patterns from this era still exist inside many large enterprises today — often invisibly, embedded in modernized wrappers. Leaders inheriting these systems underestimate their complexity at their peril. The mainframe era taught us that reliability at scale requires deliberate architecture, not just fast hardware.