But "bad" teachers pay the same in union dues as "good" teachers.
Why would the union care if a "bad" teacher was replaced with a "good" teacher? In fact, if they pay the same in dues, but the "good" teacher is less likely to need representation, the "greedy" union would make more money on a "good" teacher, right?
But, uh... er...
This has been another "Deep Reformy Thought."
(All praise to thinker in the comments, from whom I totally stole this!)