Some of that reliability is just the homogeneous software ecosystem. Where most applications developed for the AS/400 are using roughly the same stack for "ap

Some of that reliability is just the homogeneous software ecosystem. Where most applications developed for the AS/400 are using roughly the same stack for "app server", "database", and so on. That closed ecosystem means core bugs get ironed out via customer feedback pretty quickly. There's a limited amount of unique combinations of software parts and config.