Your story about the ineffective English teacher only shows that the school had an ineffective principal for a very long time. Poor teachers can be dismissed if the administrator does his/her job properly. All that is required is documentation.
And yes, the union does find it important to fight for every one of them [ineffective teachers]. Just as our justice system finds it important that even the most heinous criminals are defended. Everyone has an opportunity to be heard. If the administrator could prove that the teacher were ineffective, she would have been fired. The fact that she taught there for FIFTY (!) (really??!!) years would indicate that either she wasn’t really ineffective or that school had ineffective leadership for a very long time.
And, finally, I am a member of the National Education Association, the largest of the teacher’s unions. However, I am in a “right to work” state, so our union is rather weak. We are seeing the results in the devastating attacks on public education in this year’s legislature. Our schools would be better if we had stronger union laws.