Does making a mistake in hiring a dumbass mean that you should have to keep him on forever? Sure, there are typically ways to get rid of somebody, but the union seeks to make them as onerous as possible. The worst examples are teacher's unions whose contracts have firing procedures so convoluted, lengthy, and adversarial that principals rarely exercise them no matter how bad the teacher is.
If he is truly inept, document it and prove it. It can be done.
I have come across BAD teachers and other dumbasses. I do want them terminated. The administration is too lethargic and lazy to do their job of documentation. My son was left out-of-state on a school trip because the teacher didn't do a head count. My son was at the appointed time/place but the teacher left early because of the weather. They should have fired her.