You make it sound like everything is subjective and there are no bad or good workers. And since we can't accurately, objectively measure performance, let's just pick metric we can measure even though it has no correlation with performance.

Bad performers can absolutely be spotted, at least in the extreme. People that don't show up to work, or are chronically late (for work where this matters) or violate safety rules for example. They get protected by unions and are hard to remove.