The worst performers know they can't get an equivalent job somewhere else, so they're going to cling on no matter how miserable you make it. They have no choice

The worst performers know they can't get an equivalent job somewhere else, so they're going to cling on no matter how miserable you make it. They have no choice. Your best performers know they have their pick of any job they like so it anything gets worse for them they'll just go somewhere else.