It was true for Brooks, but Brooks worked on one of the first big software projects.<p>IBM never had a cohesive vision of a "mainframe operating system".<p>Until VM (bare metal virtual machines) was developed by academics, IBM never developed a single operating system that satisfied all the needs for mainframes. In fact, many important applications, such as CICS, ran on bare metal without a general purpose OS.<p>VM came in through the back door and became that 'one ring to rule them all'. Mainframes since 1980s are running several different operating systems for different purposes and it is all OK.