I understand that you made up your 20%, but at that point it feels like you are blaming bad team fit or environment as bad hires, and this fitting can change over time as bad politics or shifts to different goals happen.
and no i didn't make up 20%, Gartner did a study on it. and most people with management/hiring experience report the same.
"Ratings, although an important way to measure performance during a specific period, are not predictive of future performance and should not be used to gauge readiness for a future role or qualify an internal candidate for a different team. (They can, however, be used to evaluate whether an employee is properly or improperly slotted on their current team; therefore, they can provide an opportunity to evaluate how to better support an internal candidate moving forward.)"
[0] https://abseil.io/resources/swe-book/html/ch04.html#challeng...