I think true "roadblock" teachers are pretty rare, and they could be made even rarer if teacher training and ongoing support were better. Teachers are too often thrown into the fire right out of school without enough mentoring and coaching, and experienced teachers are expected to keep up with constantly changing top down expectations without much coaching either. Part of Shyamalan's prescription for success is principals who are strong leaders, and who spend their days constantly observing, interacting with, and coaching teachers in the classroom, not pushing paper behind a desk and dealing with parents and making sure the football field is lined. This is a model developed in the private charter world especially by KIPP I believe, and it ought to be more widely adopted.
At the same time, we should not pretend that there aren't some systems (NYC I know to be one) where it is damn near impossible to fire a teacher. Again, there is no meaningful teacher's union in NC or most southern states, but there is in many states and their pathological determination to keep the most incompetent and even abusive teachers on the payroll is a real problem in those places.
My personal opinion is that tenure for K-12 teachers is for the most part an outmoded concept and should be phased out. The particular way the NC legislature went about it was ham-handed and unconstitutional. The old teachers will likely get to keep their tenure unless the Pubs succeed in packing the NC Supreme Court, but even then I think they are protected under the federal constitution. In any case, new teachers coming in are going to be on contracts and ineligible for tenure. Eventually, the GA will come up with a way to constitutionally buy out the old teachers' tenure rights.