If only. With some corporate cultures there is little to no value placed on retention. In fact, as crazy as it sounds, employees are seen as a liability. This was especially obvious during the great offshoring wave of the last decade where companies couldn't wait to shed loyal-yet-expensive employees with long work histories with replaceable contract workers on the other side of the world. We all know how well that turned out.

Microsoft's (in)famous "rank&yank" comes to mind, where the employees of each department were annually scored for performance and lowest 10% were let go. Unfortunately this was copied by quite a few other companies. In this environment, absolute performance is not enough to keep one's employment, only relative performance among peers. Therefore, it is best to work with knuckleheads, easily outperformed, than with a group of geniuses where a constant struggle will exist. Perhaps it's advantageous for the company (doubtful, as even MSFT abandoned this practice) but it's highly destructive to cultures and individual's careers.

In the Susan Fowler case, at least, her manager changed a good review to a poor one to avoid losing her to another team. So I'm not sure that a blanket "if you lose too many people" policy is necessarily a good thing.

Because retention can be too subjective. If you lose a lot of bad people then retention is not a bad thing. If you start to lose good people then what was the common reason? Not willing to pay? Company culture? Team culture?

Company I work for retention can be factored in but the ability to rate your boss is given each year and if that comes back negative along with poor retention than that is reason for further action.

Why aren't managers graded?

Perhaps because it's a poor indicator of performance. There's a lot more factors at play when people quit than just "i hate my boss"

Or, grade managers on how far their direct reports advance in the company??