The truth is quite different. While it’s true that teachers who have grievances can secure the help of the union, unions go beyond that by using collective bargaining to help make sure that teachers have the conditions and materials they need to teach well.
If there really is a bad teacher, she or he can be dismissed with cause, and teacher unions just make sure that due process is followed. If there is is enough evidence that teachers have significant problems, they can still lose their jobs. No tenure policy can protect them from that.
States are trying to implement better and better evaluations so they can see what’s actually going on in the classroom, and that’s generally a good thing, so long as principals maintain their integrity in evaluating teachers. Having administrators or peers in classrooms for evals can work for more positive relationships and can help struggling teachers identify how to improve.
My experience is that there are not so many BAD teachers. We can all improve, and I’d rather see policy support improvement and change rather than punishment and censure. Teachers’ unions can help maintain enough security so that positive culture of improvement can work.